"datetime" --- Basic date and time types
****************************************

**Source code:** Lib/datetime.py

======================================================================

The "datetime" module supplies classes for manipulating dates and
times.

While date and time arithmetic is supported, the focus of the
implementation is on efficient attribute extraction for output
formatting and manipulation.

See also:

  Module "calendar"
     General calendar related functions.

  Module "time"
     Time access and conversions.

  Package dateutil
     Third-party library with expanded time zone and parsing support.


Aware and Naive Objects
=======================

Date and time objects may be categorized as "aware" or "naive"
depending on whether or not they include timezone information.

With sufficient knowledge of applicable algorithmic and political time
adjustments, such as time zone and daylight saving time information,
an **aware** object can locate itself relative to other aware objects.
An aware object represents a specific moment in time that is not open
to interpretation. [1]

A **naive** object does not contain enough information to
unambiguously locate itself relative to other date/time objects.
Whether a naive object represents Coordinated Universal Time (UTC),
local time, or time in some other timezone is purely up to the
program, just like it is up to the program whether a particular number
represents metres, miles, or mass. Naive objects are easy to
understand and to work with, at the cost of ignoring some aspects of
reality.

For applications requiring aware objects, "datetime" and "time"
objects have an optional time zone information attribute, "tzinfo",
that can be set to an instance of a subclass of the abstract "tzinfo"
class. These "tzinfo" objects capture information about the offset
from UTC time, the time zone name, and whether daylight saving time is
in effect.

Only one concrete "tzinfo" class, the "timezone" class, is supplied by
the "datetime" module. The "timezone" class can represent simple
timezones with fixed offsets from UTC, such as UTC itself or North
American EST and EDT timezones. Supporting timezones at deeper levels
of detail is up to the application. The rules for time adjustment
across the world are more political than rational, change frequently,
and there is no standard suitable for every application aside from
UTC.


Constants
=========

The "datetime" module exports the following constants:

datetime.MINYEAR

   The smallest year number allowed in a "date" or "datetime" object.
   "MINYEAR" is "1".

datetime.MAXYEAR

   The largest year number allowed in a "date" or "datetime" object.
   "MAXYEAR" is "9999".


Available Types
===============

class datetime.date

   An idealized naive date, assuming the current Gregorian calendar
   always was, and always will be, in effect. Attributes: "year",
   "month", and "day".

class datetime.time

   An idealized time, independent of any particular day, assuming that
   every day has exactly 24*60*60 seconds.  (There is no notion of
   "leap seconds" here.) Attributes: "hour", "minute", "second",
   "microsecond", and "tzinfo".

class datetime.datetime

   A combination of a date and a time. Attributes: "year", "month",
   "day", "hour", "minute", "second", "microsecond", and "tzinfo".

class datetime.timedelta

   A duration expressing the difference between two "date", "time", or
   "datetime" instances to microsecond resolution.

class datetime.tzinfo

   An abstract base class for time zone information objects. These are
   used by the "datetime" and "time" classes to provide a customizable
   notion of time adjustment (for example, to account for time zone
   and/or daylight saving time).

class datetime.timezone

   A class that implements the "tzinfo" abstract base class as a fixed
   offset from the UTC.

   New in version 3.2.

Objects of these types are immutable.

Subclass relationships:

   object
       timedelta
       tzinfo
           timezone
       time
       date
           datetime


Common Properties
-----------------

The "date", "datetime", "time", and "timezone" types share these
common features:

* Objects of these types are immutable.

* Objects of these types are hashable, meaning that they can be used
  as dictionary keys.

* Objects of these types support efficient pickling via the "pickle"
  module.


Determining if an Object is Aware or Naive
------------------------------------------

Objects of the "date" type are always naive.

An object of type "time" or "datetime" may be aware or naive.

A "datetime" object *d* is aware if both of the following hold:

1. "d.tzinfo" is not "None"

2. "d.tzinfo.utcoffset(d)" does not return "None"

Otherwise, *d* is naive.

A "time" object *t* is aware if both of the following hold:

1. "t.tzinfo" is not "None"

2. "t.tzinfo.utcoffset(None)" does not return "None".

Otherwise, *t* is naive.

The distinction between aware and naive doesn't apply to "timedelta"
objects.


"timedelta" Objects
===================

A "timedelta" object represents a duration, the difference between two
dates or times.

class datetime.timedelta(days=0, seconds=0, microseconds=0, milliseconds=0, minutes=0, hours=0, weeks=0)

   All arguments are optional and default to "0". Arguments may be
   integers or floats, and may be positive or negative.

   Only *days*, *seconds* and *microseconds* are stored internally.
   Arguments are converted to those units:

   * A millisecond is converted to 1000 microseconds.

   * A minute is converted to 60 seconds.

   * An hour is converted to 3600 seconds.

   * A week is converted to 7 days.

   and days, seconds and microseconds are then normalized so that the
   representation is unique, with

   * "0 <= microseconds < 1000000"

   * "0 <= seconds < 3600*24" (the number of seconds in one day)

   * "-999999999 <= days <= 999999999"

   The following example illustrates how any arguments besides *days*,
   *seconds* and *microseconds* are "merged" and normalized into those
   three resulting attributes:

      >>> from datetime import timedelta
      >>> delta = timedelta(
      ...     days=50,
      ...     seconds=27,
      ...     microseconds=10,
      ...     milliseconds=29000,
      ...     minutes=5,
      ...     hours=8,
      ...     weeks=2
      ... )
      >>> # Only days, seconds, and microseconds remain
      >>> delta
      datetime.timedelta(days=64, seconds=29156, microseconds=10)

   If any argument is a float and there are fractional microseconds,
   the fractional microseconds left over from all arguments are
   combined and their sum is rounded to the nearest microsecond using
   round-half-to-even tiebreaker. If no argument is a float, the
   conversion and normalization processes are exact (no information is
   lost).

   If the normalized value of days lies outside the indicated range,
   "OverflowError" is raised.

   Note that normalization of negative values may be surprising at
   first. For example:

      >>> from datetime import timedelta
      >>> d = timedelta(microseconds=-1)
      >>> (d.days, d.seconds, d.microseconds)
      (-1, 86399, 999999)

Class attributes:

timedelta.min

   The most negative "timedelta" object, "timedelta(-999999999)".

timedelta.max

   The most positive "timedelta" object, "timedelta(days=999999999,
   hours=23, minutes=59, seconds=59, microseconds=999999)".

timedelta.resolution

   The smallest possible difference between non-equal "timedelta"
   objects, "timedelta(microseconds=1)".

Note that, because of normalization, "timedelta.max" >
"-timedelta.min". "-timedelta.max" is not representable as a
"timedelta" object.

Instance attributes (read-only):

+--------------------+----------------------------------------------+
| Attribute          | Value                                        |
|====================|==============================================|
| "days"             | Between -999999999 and 999999999 inclusive   |
+--------------------+----------------------------------------------+
| "seconds"          | Between 0 and 86399 inclusive                |
+--------------------+----------------------------------------------+
| "microseconds"     | Between 0 and 999999 inclusive               |
+--------------------+----------------------------------------------+

Supported operations:

+----------------------------------+-------------------------------------------------+
| Operation                        | Result                                          |
|==================================|=================================================|
| "t1 = t2 + t3"                   | Sum of *t2* and *t3*. Afterwards *t1*-*t2* ==   |
|                                  | *t3* and *t1*-*t3* == *t2* are true. (1)        |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 - t3"                   | Difference of *t2* and *t3*. Afterwards *t1* == |
|                                  | *t2* - *t3* and *t2* == *t1* + *t3* are true.   |
|                                  | (1)(6)                                          |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 * i or t1 = i * t2"     | Delta multiplied by an integer. Afterwards *t1* |
|                                  | // i == *t2* is true, provided "i != 0".        |
+----------------------------------+-------------------------------------------------+
|                                  | In general, *t1* * i == *t1* * (i-1) + *t1* is  |
|                                  | true. (1)                                       |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 * f or t1 = f * t2"     | Delta multiplied by a float. The result is      |
|                                  | rounded to the nearest multiple of              |
|                                  | timedelta.resolution using round-half-to-even.  |
+----------------------------------+-------------------------------------------------+
| "f = t2 / t3"                    | Division (3) of overall duration *t2* by        |
|                                  | interval unit *t3*. Returns a "float" object.   |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 / f or t1 = t2 / i"     | Delta divided by a float or an int. The result  |
|                                  | is rounded to the nearest multiple of           |
|                                  | timedelta.resolution using round-half-to-even.  |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 // i" or "t1 = t2 //    | The floor is computed and the remainder (if     |
| t3"                              | any) is thrown away. In the second case, an     |
|                                  | integer is returned. (3)                        |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 % t3"                   | The remainder is computed as a "timedelta"      |
|                                  | object. (3)                                     |
+----------------------------------+-------------------------------------------------+
| "q, r = divmod(t1, t2)"          | Computes the quotient and the remainder: "q =   |
|                                  | t1 // t2" (3) and "r = t1 % t2". q is an        |
|                                  | integer and r is a "timedelta" object.          |
+----------------------------------+-------------------------------------------------+
| "+t1"                            | Returns a "timedelta" object with the same      |
|                                  | value. (2)                                      |
+----------------------------------+-------------------------------------------------+
| "-t1"                            | equivalent to "timedelta"(-*t1.days*,           |
|                                  | -*t1.seconds*, -*t1.microseconds*), and to      |
|                                  | *t1** -1. (1)(4)                                |
+----------------------------------+-------------------------------------------------+
| "abs(t)"                         | equivalent to +*t* when "t.days >= 0", and to   |
|                                  | -*t* when "t.days < 0". (2)                     |
+----------------------------------+-------------------------------------------------+
| "str(t)"                         | Returns a string in the form "[D day[s],        |
|                                  | ][H]H:MM:SS[.UUUUUU]", where D is negative for  |
|                                  | negative "t". (5)                               |
+----------------------------------+-------------------------------------------------+
| "repr(t)"                        | Returns a string representation of the          |
|                                  | "timedelta" object as a constructor call with   |
|                                  | canonical attribute values.                     |
+----------------------------------+-------------------------------------------------+

Notes:

1. This is exact but may overflow.

2. This is exact and cannot overflow.

3. Division by 0 raises "ZeroDivisionError".

4. -*timedelta.max* is not representable as a "timedelta" object.

5. String representations of "timedelta" objects are normalized
   similarly to their internal representation. This leads to somewhat
   unusual results for negative timedeltas. For example:

      >>> timedelta(hours=-5)
      datetime.timedelta(days=-1, seconds=68400)
      >>> print(_)
      -1 day, 19:00:00

6. The expression "t2 - t3" will always be equal to the expression "t2
   + (-t3)" except when t3 is equal to "timedelta.max"; in that case
   the former will produce a result while the latter will overflow.

In addition to the operations listed above, "timedelta" objects
support certain additions and subtractions with "date" and "datetime"
objects (see below).

Changed in version 3.2: Floor division and true division of a
"timedelta" object by another "timedelta" object are now supported, as
are remainder operations and the "divmod()" function. True division
and multiplication of a "timedelta" object by a "float" object are now
supported.

Comparisons of "timedelta" objects are supported, with some caveats.

The comparisons "==" or "!=" *always* return a "bool", no matter the
type of the compared object:

   >>> from datetime import timedelta
   >>> delta1 = timedelta(seconds=57)
   >>> delta2 = timedelta(hours=25, seconds=2)
   >>> delta2 != delta1
   True
   >>> delta2 == 5
   False

For all other comparisons (such as "<" and ">"), when a "timedelta"
object is compared to an object of a different type, "TypeError" is
raised:

   >>> delta2 > delta1
   True
   >>> delta2 > 5
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: '>' not supported between instances of 'datetime.timedelta' and 'int'

In Boolean contexts, a "timedelta" object is considered to be true if
and only if it isn't equal to "timedelta(0)".

Instance methods:

timedelta.total_seconds()

   Return the total number of seconds contained in the duration.
   Equivalent to "td / timedelta(seconds=1)". For interval units other
   than seconds, use the division form directly (e.g. "td /
   timedelta(microseconds=1)").

   Note that for very large time intervals (greater than 270 years on
   most platforms) this method will lose microsecond accuracy.

   New in version 3.2.


Examples of usage: "timedelta"
------------------------------

An additional example of normalization:

   >>> # Components of another_year add up to exactly 365 days
   >>> from datetime import timedelta
   >>> year = timedelta(days=365)
   >>> another_year = timedelta(weeks=40, days=84, hours=23,
   ...                          minutes=50, seconds=600)
   >>> year == another_year
   True
   >>> year.total_seconds()
   31536000.0

Examples of "timedelta" arithmetic:

   >>> from datetime import timedelta
   >>> year = timedelta(days=365)
   >>> ten_years = 10 * year
   >>> ten_years
   datetime.timedelta(days=3650)
   >>> ten_years.days // 365
   10
   >>> nine_years = ten_years - year
   >>> nine_years
   datetime.timedelta(days=3285)
   >>> three_years = nine_years // 3
   >>> three_years, three_years.days // 365
   (datetime.timedelta(days=1095), 3)


"date" Objects
==============

A "date" object represents a date (year, month and day) in an
idealized calendar, the current Gregorian calendar indefinitely
extended in both directions.

January 1 of year 1 is called day number 1, January 2 of year 1 is
called day number 2, and so on. [2]

class datetime.date(year, month, day)

   All arguments are required. Arguments must be integers, in the
   following ranges:

   * "MINYEAR <= year <= MAXYEAR"

   * "1 <= month <= 12"

   * "1 <= day <= number of days in the given month and year"

   If an argument outside those ranges is given, "ValueError" is
   raised.

Other constructors, all class methods:

classmethod date.today()

   Return the current local date.

   This is equivalent to "date.fromtimestamp(time.time())".

classmethod date.fromtimestamp(timestamp)

   Return the local date corresponding to the POSIX timestamp, such as
   is returned by "time.time()".

   This may raise "OverflowError", if the timestamp is out of the
   range of values supported by the platform C "localtime()" function,
   and "OSError" on "localtime()" failure. It's common for this to be
   restricted to years from 1970 through 2038. Note that on non-POSIX
   systems that include leap seconds in their notion of a timestamp,
   leap seconds are ignored by "fromtimestamp()".

   Changed in version 3.3: Raise "OverflowError" instead of
   "ValueError" if the timestamp is out of the range of values
   supported by the platform C "localtime()" function. Raise "OSError"
   instead of "ValueError" on "localtime()" failure.

classmethod date.fromordinal(ordinal)

   Return the date corresponding to the proleptic Gregorian ordinal,
   where January 1 of year 1 has ordinal 1.

   "ValueError" is raised unless "1 <= ordinal <=
   date.max.toordinal()". For any date *d*,
   "date.fromordinal(d.toordinal()) == d".

classmethod date.fromisoformat(date_string)

   Return a "date" corresponding to a *date_string* given in the
   format "YYYY-MM-DD":

      >>> from datetime import date
      >>> date.fromisoformat('2019-12-04')
      datetime.date(2019, 12, 4)

   This is the inverse of "date.isoformat()". It only supports the
   format "YYYY-MM-DD".

   New in version 3.7.

classmethod date.fromisocalendar(year, week, day)

   Return a "date" corresponding to the ISO calendar date specified by
   year, week and day. This is the inverse of the function
   "date.isocalendar()".

   New in version 3.8.

Class attributes:

date.min

   The earliest representable date, "date(MINYEAR, 1, 1)".

date.max

   The latest representable date, "date(MAXYEAR, 12, 31)".

date.resolution

   The smallest possible difference between non-equal date objects,
   "timedelta(days=1)".

Instance attributes (read-only):

date.year

   Between "MINYEAR" and "MAXYEAR" inclusive.

date.month

   Between 1 and 12 inclusive.

date.day

   Between 1 and the number of days in the given month of the given
   year.

Supported operations:

+---------------------------------+------------------------------------------------+
| Operation                       | Result                                         |
|=================================|================================================|
| "date2 = date1 + timedelta"     | *date2* is "timedelta.days" days removed from  |
|                                 | *date1*. (1)                                   |
+---------------------------------+------------------------------------------------+
| "date2 = date1 - timedelta"     | Computes *date2* such that "date2 + timedelta  |
|                                 | == date1". (2)                                 |
+---------------------------------+------------------------------------------------+
| "timedelta = date1 - date2"     | (3)                                            |
+---------------------------------+------------------------------------------------+
| "date1 < date2"                 | *date1* is considered less than *date2* when   |
|                                 | *date1* precedes *date2* in time. (4)          |
+---------------------------------+------------------------------------------------+

Notes:

1. *date2* is moved forward in time if "timedelta.days > 0", or
   backward if "timedelta.days < 0". Afterward "date2 - date1 ==
   timedelta.days". "timedelta.seconds" and "timedelta.microseconds"
   are ignored. "OverflowError" is raised if "date2.year" would be
   smaller than "MINYEAR" or larger than "MAXYEAR".

2. "timedelta.seconds" and "timedelta.microseconds" are ignored.

3. This is exact, and cannot overflow. timedelta.seconds and
   timedelta.microseconds are 0, and date2 + timedelta == date1 after.

4. In other words, "date1 < date2" if and only if "date1.toordinal() <
   date2.toordinal()". Date comparison raises "TypeError" if the other
   comparand isn't also a "date" object. However, "NotImplemented" is
   returned instead if the other comparand has a "timetuple()"
   attribute. This hook gives other kinds of date objects a chance at
   implementing mixed-type comparison. If not, when a "date" object is
   compared to an object of a different type, "TypeError" is raised
   unless the comparison is "==" or "!=". The latter cases return
   "False" or "True", respectively.

In Boolean contexts, all "date" objects are considered to be true.

Instance methods:

date.replace(year=self.year, month=self.month, day=self.day)

   Return a date with the same value, except for those parameters
   given new values by whichever keyword arguments are specified.

   Example:

      >>> from datetime import date
      >>> d = date(2002, 12, 31)
      >>> d.replace(day=26)
      datetime.date(2002, 12, 26)

date.timetuple()

   Return a "time.struct_time" such as returned by "time.localtime()".

   The hours, minutes and seconds are 0, and the DST flag is -1.

   "d.timetuple()" is equivalent to:

      time.struct_time((d.year, d.month, d.day, 0, 0, 0, d.weekday(), yday, -1))

   where "yday = d.toordinal() - date(d.year, 1, 1).toordinal() + 1"
   is the day number within the current year starting with "1" for
   January 1st.

date.toordinal()

   Return the proleptic Gregorian ordinal of the date, where January 1
   of year 1 has ordinal 1. For any "date" object *d*,
   "date.fromordinal(d.toordinal()) == d".

date.weekday()

   Return the day of the week as an integer, where Monday is 0 and
   Sunday is 6. For example, "date(2002, 12, 4).weekday() == 2", a
   Wednesday. See also "isoweekday()".

date.isoweekday()

   Return the day of the week as an integer, where Monday is 1 and
   Sunday is 7. For example, "date(2002, 12, 4).isoweekday() == 3", a
   Wednesday. See also "weekday()", "isocalendar()".

date.isocalendar()

   Return a *named tuple* object with three components: "year", "week"
   and "weekday".

   The ISO calendar is a widely used variant of the Gregorian
   calendar. [3]

   The ISO year consists of 52 or 53 full weeks, and where a week
   starts on a Monday and ends on a Sunday. The first week of an ISO
   year is the first (Gregorian) calendar week of a year containing a
   Thursday. This is called week number 1, and the ISO year of that
   Thursday is the same as its Gregorian year.

   For example, 2004 begins on a Thursday, so the first week of ISO
   year 2004 begins on Monday, 29 Dec 2003 and ends on Sunday, 4 Jan
   2004:

      >>> from datetime import date
      >>> date(2003, 12, 29).isocalendar()
      datetime.IsoCalendarDate(year=2004, week=1, weekday=1)
      >>> date(2004, 1, 4).isocalendar()
      datetime.IsoCalendarDate(year=2004, week=1, weekday=7)

   Changed in version 3.9: Result changed from a tuple to a *named
   tuple*.

date.isoformat()

   Return a string representing the date in ISO 8601 format, "YYYY-MM-
   DD":

      >>> from datetime import date
      >>> date(2002, 12, 4).isoformat()
      '2002-12-04'

   This is the inverse of "date.fromisoformat()".

date.__str__()

   For a date *d*, "str(d)" is equivalent to "d.isoformat()".

date.ctime()

   Return a string representing the date:

      >>> from datetime import date
      >>> date(2002, 12, 4).ctime()
      'Wed Dec  4 00:00:00 2002'

   "d.ctime()" is equivalent to:

      time.ctime(time.mktime(d.timetuple()))

   on platforms where the native C "ctime()" function (which
   "time.ctime()" invokes, but which "date.ctime()" does not invoke)
   conforms to the C standard.

date.strftime(format)

   Return a string representing the date, controlled by an explicit
   format string. Format codes referring to hours, minutes or seconds
   will see 0 values. For a complete list of formatting directives,
   see strftime() and strptime() Behavior.

date.__format__(format)

   Same as "date.strftime()". This makes it possible to specify a
   format string for a "date" object in formatted string literals and
   when using "str.format()". For a complete list of formatting
   directives, see strftime() and strptime() Behavior.


Examples of Usage: "date"
-------------------------

Example of counting days to an event:

   >>> import time
   >>> from datetime import date
   >>> today = date.today()
   >>> today
   datetime.date(2007, 12, 5)
   >>> today == date.fromtimestamp(time.time())
   True
   >>> my_birthday = date(today.year, 6, 24)
   >>> if my_birthday < today:
   ...     my_birthday = my_birthday.replace(year=today.year + 1)
   >>> my_birthday
   datetime.date(2008, 6, 24)
   >>> time_to_birthday = abs(my_birthday - today)
   >>> time_to_birthday.days
   202

More examples of working with "date":

   >>> from datetime import date
   >>> d = date.fromordinal(730920) # 730920th day after 1. 1. 0001
   >>> d
   datetime.date(2002, 3, 11)

   >>> # Methods related to formatting string output
   >>> d.isoformat()
   '2002-03-11'
   >>> d.strftime("%d/%m/%y")
   '11/03/02'
   >>> d.strftime("%A %d. %B %Y")
   'Monday 11. March 2002'
   >>> d.ctime()
   'Mon Mar 11 00:00:00 2002'
   >>> 'The {1} is {0:%d}, the {2} is {0:%B}.'.format(d, "day", "month")
   'The day is 11, the month is March.'

   >>> # Methods for to extracting 'components' under different calendars
   >>> t = d.timetuple()
   >>> for i in t:     
   ...     print(i)
   2002                # year
   3                   # month
   11                  # day
   0
   0
   0
   0                   # weekday (0 = Monday)
   70                  # 70th day in the year
   -1
   >>> ic = d.isocalendar()
   >>> for i in ic:    
   ...     print(i)
   2002                # ISO year
   11                  # ISO week number
   1                   # ISO day number ( 1 = Monday )

   >>> # A date object is immutable; all operations produce a new object
   >>> d.replace(year=2005)
   datetime.date(2005, 3, 11)


"datetime" Objects
==================

A "datetime" object is a single object containing all the information
from a "date" object and a "time" object.

Like a "date" object, "datetime" assumes the current Gregorian
calendar extended in both directions; like a "time" object, "datetime"
assumes there are exactly 3600*24 seconds in every day.

Constructor:

class datetime.datetime(year, month, day, hour=0, minute=0, second=0, microsecond=0, tzinfo=None, *, fold=0)

   The *year*, *month* and *day* arguments are required. *tzinfo* may
   be "None", or an instance of a "tzinfo" subclass. The remaining
   arguments must be integers in the following ranges:

   * "MINYEAR <= year <= MAXYEAR",

   * "1 <= month <= 12",

   * "1 <= day <= number of days in the given month and year",

   * "0 <= hour < 24",

   * "0 <= minute < 60",

   * "0 <= second < 60",

   * "0 <= microsecond < 1000000",

   * "fold in [0, 1]".

   If an argument outside those ranges is given, "ValueError" is
   raised.

   New in version 3.6: Added the "fold" argument.

Other constructors, all class methods:

classmethod datetime.today()

   Return the current local datetime, with "tzinfo" "None".

   Equivalent to:

      datetime.fromtimestamp(time.time())

   See also "now()", "fromtimestamp()".

   This method is functionally equivalent to "now()", but without a
   "tz" parameter.

classmethod datetime.now(tz=None)

   Return the current local date and time.

   If optional argument *tz* is "None" or not specified, this is like
   "today()", but, if possible, supplies more precision than can be
   gotten from going through a "time.time()" timestamp (for example,
   this may be possible on platforms supplying the C "gettimeofday()"
   function).

   If *tz* is not "None", it must be an instance of a "tzinfo"
   subclass, and the current date and time are converted to *tz*’s
   time zone.

   This function is preferred over "today()" and "utcnow()".

classmethod datetime.utcnow()

   Return the current UTC date and time, with "tzinfo" "None".

   This is like "now()", but returns the current UTC date and time, as
   a naive "datetime" object. An aware current UTC datetime can be
   obtained by calling "datetime.now(timezone.utc)". See also "now()".

   Warning:

     Because naive "datetime" objects are treated by many "datetime"
     methods as local times, it is preferred to use aware datetimes to
     represent times in UTC. As such, the recommended way to create an
     object representing the current time in UTC is by calling
     "datetime.now(timezone.utc)".

classmethod datetime.fromtimestamp(timestamp, tz=None)

   Return the local date and time corresponding to the POSIX
   timestamp, such as is returned by "time.time()". If optional
   argument *tz* is "None" or not specified, the timestamp is
   converted to the platform's local date and time, and the returned
   "datetime" object is naive.

   If *tz* is not "None", it must be an instance of a "tzinfo"
   subclass, and the timestamp is converted to *tz*’s time zone.

   "fromtimestamp()" may raise "OverflowError", if the timestamp is
   out of the range of values supported by the platform C
   "localtime()" or "gmtime()" functions, and "OSError" on
   "localtime()" or "gmtime()" failure. It's common for this to be
   restricted to years in 1970 through 2038. Note that on non-POSIX
   systems that include leap seconds in their notion of a timestamp,
   leap seconds are ignored by "fromtimestamp()", and then it's
   possible to have two timestamps differing by a second that yield
   identical "datetime" objects. This method is preferred over
   "utcfromtimestamp()".

   Changed in version 3.3: Raise "OverflowError" instead of
   "ValueError" if the timestamp is out of the range of values
   supported by the platform C "localtime()" or "gmtime()" functions.
   Raise "OSError" instead of "ValueError" on "localtime()" or
   "gmtime()" failure.

   Changed in version 3.6: "fromtimestamp()" may return instances with
   "fold" set to 1.

classmethod datetime.utcfromtimestamp(timestamp)

   Return the UTC "datetime" corresponding to the POSIX timestamp,
   with "tzinfo" "None".  (The resulting object is naive.)

   This may raise "OverflowError", if the timestamp is out of the
   range of values supported by the platform C "gmtime()" function,
   and "OSError" on "gmtime()" failure. It's common for this to be
   restricted to years in 1970 through 2038.

   To get an aware "datetime" object, call "fromtimestamp()":

      datetime.fromtimestamp(timestamp, timezone.utc)

   On the POSIX compliant platforms, it is equivalent to the following
   expression:

      datetime(1970, 1, 1, tzinfo=timezone.utc) + timedelta(seconds=timestamp)

   except the latter formula always supports the full years range:
   between "MINYEAR" and "MAXYEAR" inclusive.

   Warning:

     Because naive "datetime" objects are treated by many "datetime"
     methods as local times, it is preferred to use aware datetimes to
     represent times in UTC. As such, the recommended way to create an
     object representing a specific timestamp in UTC is by calling
     "datetime.fromtimestamp(timestamp, tz=timezone.utc)".

   Changed in version 3.3: Raise "OverflowError" instead of
   "ValueError" if the timestamp is out of the range of values
   supported by the platform C "gmtime()" function. Raise "OSError"
   instead of "ValueError" on "gmtime()" failure.

classmethod datetime.fromordinal(ordinal)

   Return the "datetime" corresponding to the proleptic Gregorian
   ordinal, where January 1 of year 1 has ordinal 1. "ValueError" is
   raised unless "1 <= ordinal <= datetime.max.toordinal()". The hour,
   minute, second and microsecond of the result are all 0, and
   "tzinfo" is "None".

classmethod datetime.combine(date, time, tzinfo=self.tzinfo)

   Return a new "datetime" object whose date components are equal to
   the given "date" object's, and whose time components are equal to
   the given "time" object's. If the *tzinfo* argument is provided,
   its value is used to set the "tzinfo" attribute of the result,
   otherwise the "tzinfo" attribute of the *time* argument is used.

   For any "datetime" object *d*, "d == datetime.combine(d.date(),
   d.time(), d.tzinfo)". If date is a "datetime" object, its time
   components and "tzinfo" attributes are ignored.

   Changed in version 3.6: Added the *tzinfo* argument.

classmethod datetime.fromisoformat(date_string)

   Return a "datetime" corresponding to a *date_string* in one of the
   formats emitted by "date.isoformat()" and "datetime.isoformat()".

   Specifically, this function supports strings in the format:

      YYYY-MM-DD[*HH[:MM[:SS[.fff[fff]]]][+HH:MM[:SS[.ffffff]]]]

   where "*" can match any single character.

   Caution:

     This does *not* support parsing arbitrary ISO 8601 strings - it
     is only intended as the inverse operation of
     "datetime.isoformat()". A more full-featured ISO 8601 parser,
     "dateutil.parser.isoparse" is available in the third-party
     package dateutil.

   Examples:

      >>> from datetime import datetime
      >>> datetime.fromisoformat('2011-11-04')
      datetime.datetime(2011, 11, 4, 0, 0)
      >>> datetime.fromisoformat('2011-11-04T00:05:23')
      datetime.datetime(2011, 11, 4, 0, 5, 23)
      >>> datetime.fromisoformat('2011-11-04 00:05:23.283')
      datetime.datetime(2011, 11, 4, 0, 5, 23, 283000)
      >>> datetime.fromisoformat('2011-11-04 00:05:23.283+00:00')
      datetime.datetime(2011, 11, 4, 0, 5, 23, 283000, tzinfo=datetime.timezone.utc)
      >>> datetime.fromisoformat('2011-11-04T00:05:23+04:00')   
      datetime.datetime(2011, 11, 4, 0, 5, 23,
          tzinfo=datetime.timezone(datetime.timedelta(seconds=14400)))

   New in version 3.7.

classmethod datetime.fromisocalendar(year, week, day)

   Return a "datetime" corresponding to the ISO calendar date
   specified by year, week and day. The non-date components of the
   datetime are populated with their normal default values. This is
   the inverse of the function "datetime.isocalendar()".

   New in version 3.8.

classmethod datetime.strptime(date_string, format)

   Return a "datetime" corresponding to *date_string*, parsed
   according to *format*.

   This is equivalent to:

      datetime(*(time.strptime(date_string, format)[0:6]))

   "ValueError" is raised if the date_string and format can't be
   parsed by "time.strptime()" or if it returns a value which isn't a
   time tuple. For a complete list of formatting directives, see
   strftime() and strptime() Behavior.

Class attributes:

datetime.min

   The earliest representable "datetime", "datetime(MINYEAR, 1, 1,
   tzinfo=None)".

datetime.max

   The latest representable "datetime", "datetime(MAXYEAR, 12, 31, 23,
   59, 59, 999999, tzinfo=None)".

datetime.resolution

   The smallest possible difference between non-equal "datetime"
   objects, "timedelta(microseconds=1)".

Instance attributes (read-only):

datetime.year

   Between "MINYEAR" and "MAXYEAR" inclusive.

datetime.month

   Between 1 and 12 inclusive.

datetime.day

   Between 1 and the number of days in the given month of the given
   year.

datetime.hour

   In "range(24)".

datetime.minute

   In "range(60)".

datetime.second

   In "range(60)".

datetime.microsecond

   In "range(1000000)".

datetime.tzinfo

   The object passed as the *tzinfo* argument to the "datetime"
   constructor, or "None" if none was passed.

datetime.fold

   In "[0, 1]". Used to disambiguate wall times during a repeated
   interval. (A repeated interval occurs when clocks are rolled back
   at the end of daylight saving time or when the UTC offset for the
   current zone is decreased for political reasons.) The value 0 (1)
   represents the earlier (later) of the two moments with the same
   wall time representation.

   New in version 3.6.

Supported operations:

+-----------------------------------------+----------------------------------+
| Operation                               | Result                           |
|=========================================|==================================|
| "datetime2 = datetime1 + timedelta"     | (1)                              |
+-----------------------------------------+----------------------------------+
| "datetime2 = datetime1 - timedelta"     | (2)                              |
+-----------------------------------------+----------------------------------+
| "timedelta = datetime1 - datetime2"     | (3)                              |
+-----------------------------------------+----------------------------------+
| "datetime1 < datetime2"                 | Compares "datetime" to           |
|                                         | "datetime". (4)                  |
+-----------------------------------------+----------------------------------+

1. datetime2 is a duration of timedelta removed from datetime1, moving
   forward in time if "timedelta.days" > 0, or backward if
   "timedelta.days" < 0. The result has the same "tzinfo" attribute as
   the input datetime, and datetime2 - datetime1 == timedelta after.
   "OverflowError" is raised if datetime2.year would be smaller than
   "MINYEAR" or larger than "MAXYEAR". Note that no time zone
   adjustments are done even if the input is an aware object.

2. Computes the datetime2 such that datetime2 + timedelta ==
   datetime1. As for addition, the result has the same "tzinfo"
   attribute as the input datetime, and no time zone adjustments are
   done even if the input is aware.

3. Subtraction of a "datetime" from a "datetime" is defined only if
   both operands are naive, or if both are aware. If one is aware and
   the other is naive, "TypeError" is raised.

   If both are naive, or both are aware and have the same "tzinfo"
   attribute, the "tzinfo" attributes are ignored, and the result is a
   "timedelta" object *t* such that "datetime2 + t == datetime1". No
   time zone adjustments are done in this case.

   If both are aware and have different "tzinfo" attributes, "a-b"
   acts as if *a* and *b* were first converted to naive UTC datetimes
   first. The result is "(a.replace(tzinfo=None) - a.utcoffset()) -
   (b.replace(tzinfo=None) - b.utcoffset())" except that the
   implementation never overflows.

4. *datetime1* is considered less than *datetime2* when *datetime1*
   precedes *datetime2* in time.

   If one comparand is naive and the other is aware, "TypeError" is
   raised if an order comparison is attempted. For equality
   comparisons, naive instances are never equal to aware instances.

   If both comparands are aware, and have the same "tzinfo" attribute,
   the common "tzinfo" attribute is ignored and the base datetimes are
   compared. If both comparands are aware and have different "tzinfo"
   attributes, the comparands are first adjusted by subtracting their
   UTC offsets (obtained from "self.utcoffset()").

   Changed in version 3.3: Equality comparisons between aware and
   naive "datetime" instances don't raise "TypeError".

   Note:

     In order to stop comparison from falling back to the default
     scheme of comparing object addresses, datetime comparison
     normally raises "TypeError" if the other comparand isn't also a
     "datetime" object. However, "NotImplemented" is returned instead
     if the other comparand has a "timetuple()" attribute. This hook
     gives other kinds of date objects a chance at implementing mixed-
     type comparison. If not, when a "datetime" object is compared to
     an object of a different type, "TypeError" is raised unless the
     comparison is "==" or "!=". The latter cases return "False" or
     "True", respectively.

Instance methods:

datetime.date()

   Return "date" object with same year, month and day.

datetime.time()

   Return "time" object with same hour, minute, second, microsecond
   and fold. "tzinfo" is "None". See also method "timetz()".

   Changed in version 3.6: The fold value is copied to the returned
   "time" object.

datetime.timetz()

   Return "time" object with same hour, minute, second, microsecond,
   fold, and tzinfo attributes. See also method "time()".

   Changed in version 3.6: The fold value is copied to the returned
   "time" object.

datetime.replace(year=self.year, month=self.month, day=self.day, hour=self.hour, minute=self.minute, second=self.second, microsecond=self.microsecond, tzinfo=self.tzinfo, *, fold=0)

   Return a datetime with the same attributes, except for those
   attributes given new values by whichever keyword arguments are
   specified. Note that "tzinfo=None" can be specified to create a
   naive datetime from an aware datetime with no conversion of date
   and time data.

   New in version 3.6: Added the "fold" argument.

datetime.astimezone(tz=None)

   Return a "datetime" object with new "tzinfo" attribute *tz*,
   adjusting the date and time data so the result is the same UTC time
   as *self*, but in *tz*'s local time.

   If provided, *tz* must be an instance of a "tzinfo" subclass, and
   its "utcoffset()" and "dst()" methods must not return "None". If
   *self* is naive, it is presumed to represent time in the system
   timezone.

   If called without arguments (or with "tz=None") the system local
   timezone is assumed for the target timezone. The ".tzinfo"
   attribute of the converted datetime instance will be set to an
   instance of "timezone" with the zone name and offset obtained from
   the OS.

   If "self.tzinfo" is *tz*, "self.astimezone(tz)" is equal to *self*:
   no adjustment of date or time data is performed. Else the result is
   local time in the timezone *tz*, representing the same UTC time as
   *self*:  after "astz = dt.astimezone(tz)", "astz -
   astz.utcoffset()" will have the same date and time data as "dt -
   dt.utcoffset()".

   If you merely want to attach a time zone object *tz* to a datetime
   *dt* without adjustment of date and time data, use
   "dt.replace(tzinfo=tz)". If you merely want to remove the time zone
   object from an aware datetime *dt* without conversion of date and
   time data, use "dt.replace(tzinfo=None)".

   Note that the default "tzinfo.fromutc()" method can be overridden
   in a "tzinfo" subclass to affect the result returned by
   "astimezone()". Ignoring error cases, "astimezone()" acts like:

      def astimezone(self, tz):
          if self.tzinfo is tz:
              return self
          # Convert self to UTC, and attach the new time zone object.
          utc = (self - self.utcoffset()).replace(tzinfo=tz)
          # Convert from UTC to tz's local time.
          return tz.fromutc(utc)

   Changed in version 3.3: *tz* now can be omitted.

   Changed in version 3.6: The "astimezone()" method can now be called
   on naive instances that are presumed to represent system local
   time.

datetime.utcoffset()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.utcoffset(self)", and raises an exception if the
   latter doesn't return "None" or a "timedelta" object with magnitude
   less than one day.

   Changed in version 3.7: The UTC offset is not restricted to a whole
   number of minutes.

datetime.dst()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.dst(self)", and raises an exception if the latter
   doesn't return "None" or a "timedelta" object with magnitude less
   than one day.

   Changed in version 3.7: The DST offset is not restricted to a whole
   number of minutes.

datetime.tzname()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.tzname(self)", raises an exception if the latter
   doesn't return "None" or a string object,

datetime.timetuple()

   Return a "time.struct_time" such as returned by "time.localtime()".

   "d.timetuple()" is equivalent to:

      time.struct_time((d.year, d.month, d.day,
                        d.hour, d.minute, d.second,
                        d.weekday(), yday, dst))

   where "yday = d.toordinal() - date(d.year, 1, 1).toordinal() + 1"
   is the day number within the current year starting with "1" for
   January 1st. The "tm_isdst" flag of the result is set according to
   the "dst()" method: "tzinfo" is "None" or "dst()" returns "None",
   "tm_isdst" is set to "-1"; else if "dst()" returns a non-zero
   value, "tm_isdst" is set to "1"; else "tm_isdst" is set to "0".

datetime.utctimetuple()

   If "datetime" instance *d* is naive, this is the same as
   "d.timetuple()" except that "tm_isdst" is forced to 0 regardless of
   what "d.dst()" returns. DST is never in effect for a UTC time.

   If *d* is aware, *d* is normalized to UTC time, by subtracting
   "d.utcoffset()", and a "time.struct_time" for the normalized time
   is returned. "tm_isdst" is forced to 0. Note that an
   "OverflowError" may be raised if *d*.year was "MINYEAR" or
   "MAXYEAR" and UTC adjustment spills over a year boundary.

   Warning:

     Because naive "datetime" objects are treated by many "datetime"
     methods as local times, it is preferred to use aware datetimes to
     represent times in UTC; as a result, using "utcfromtimetuple" may
     give misleading results. If you have a naive "datetime"
     representing UTC, use "datetime.replace(tzinfo=timezone.utc)" to
     make it aware, at which point you can use "datetime.timetuple()".

datetime.toordinal()

   Return the proleptic Gregorian ordinal of the date. The same as
   "self.date().toordinal()".

datetime.timestamp()

   Return POSIX timestamp corresponding to the "datetime" instance.
   The return value is a "float" similar to that returned by
   "time.time()".

   Naive "datetime" instances are assumed to represent local time and
   this method relies on the platform C "mktime()" function to perform
   the conversion. Since "datetime" supports wider range of values
   than "mktime()" on many platforms, this method may raise
   "OverflowError" for times far in the past or far in the future.

   For aware "datetime" instances, the return value is computed as:

      (dt - datetime(1970, 1, 1, tzinfo=timezone.utc)).total_seconds()

   New in version 3.3.

   Changed in version 3.6: The "timestamp()" method uses the "fold"
   attribute to disambiguate the times during a repeated interval.

   Note:

     There is no method to obtain the POSIX timestamp directly from a
     naive "datetime" instance representing UTC time. If your
     application uses this convention and your system timezone is not
     set to UTC, you can obtain the POSIX timestamp by supplying
     "tzinfo=timezone.utc":

        timestamp = dt.replace(tzinfo=timezone.utc).timestamp()

     or by calculating the timestamp directly:

        timestamp = (dt - datetime(1970, 1, 1)) / timedelta(seconds=1)

datetime.weekday()

   Return the day of the week as an integer, where Monday is 0 and
   Sunday is 6. The same as "self.date().weekday()". See also
   "isoweekday()".

datetime.isoweekday()

   Return the day of the week as an integer, where Monday is 1 and
   Sunday is 7. The same as "self.date().isoweekday()". See also
   "weekday()", "isocalendar()".

datetime.isocalendar()

   Return a *named tuple* with three components: "year", "week" and
   "weekday". The same as "self.date().isocalendar()".

datetime.isoformat(sep='T', timespec='auto')

   Return a string representing the date and time in ISO 8601 format:

   * "YYYY-MM-DDTHH:MM:SS.ffffff", if "microsecond" is not 0

   * "YYYY-MM-DDTHH:MM:SS", if "microsecond" is 0

   If "utcoffset()" does not return "None", a string is appended,
   giving the UTC offset:

   * "YYYY-MM-DDTHH:MM:SS.ffffff+HH:MM[:SS[.ffffff]]", if
     "microsecond" is not 0

   * "YYYY-MM-DDTHH:MM:SS+HH:MM[:SS[.ffffff]]",  if "microsecond" is 0

   Examples:

      >>> from datetime import datetime, timezone
      >>> datetime(2019, 5, 18, 15, 17, 8, 132263).isoformat()
      '2019-05-18T15:17:08.132263'
      >>> datetime(2019, 5, 18, 15, 17, tzinfo=timezone.utc).isoformat()
      '2019-05-18T15:17:00+00:00'

   The optional argument *sep* (default "'T'") is a one-character
   separator, placed between the date and time portions of the result.
   For example:

      >>> from datetime import tzinfo, timedelta, datetime
      >>> class TZ(tzinfo):
      ...     """A time zone with an arbitrary, constant -06:39 offset."""
      ...     def utcoffset(self, dt):
      ...         return timedelta(hours=-6, minutes=-39)
      ...
      >>> datetime(2002, 12, 25, tzinfo=TZ()).isoformat(' ')
      '2002-12-25 00:00:00-06:39'
      >>> datetime(2009, 11, 27, microsecond=100, tzinfo=TZ()).isoformat()
      '2009-11-27T00:00:00.000100-06:39'

   The optional argument *timespec* specifies the number of additional
   components of the time to include (the default is "'auto'"). It can
   be one of the following:

   * "'auto'": Same as "'seconds'" if "microsecond" is 0, same as
     "'microseconds'" otherwise.

   * "'hours'": Include the "hour" in the two-digit "HH" format.

   * "'minutes'": Include "hour" and "minute" in "HH:MM" format.

   * "'seconds'": Include "hour", "minute", and "second" in "HH:MM:SS"
     format.

   * "'milliseconds'": Include full time, but truncate fractional
     second part to milliseconds. "HH:MM:SS.sss" format.

   * "'microseconds'": Include full time in "HH:MM:SS.ffffff" format.

   Note:

     Excluded time components are truncated, not rounded.

   "ValueError" will be raised on an invalid *timespec* argument:

      >>> from datetime import datetime
      >>> datetime.now().isoformat(timespec='minutes')   
      '2002-12-25T00:00'
      >>> dt = datetime(2015, 1, 1, 12, 30, 59, 0)
      >>> dt.isoformat(timespec='microseconds')
      '2015-01-01T12:30:59.000000'

   New in version 3.6: Added the *timespec* argument.

datetime.__str__()

   For a "datetime" instance *d*, "str(d)" is equivalent to
   "d.isoformat(' ')".

datetime.ctime()

   Return a string representing the date and time:

      >>> from datetime import datetime
      >>> datetime(2002, 12, 4, 20, 30, 40).ctime()
      'Wed Dec  4 20:30:40 2002'

   The output string will *not* include time zone information,
   regardless of whether the input is aware or naive.

   "d.ctime()" is equivalent to:

      time.ctime(time.mktime(d.timetuple()))

   on platforms where the native C "ctime()" function (which
   "time.ctime()" invokes, but which "datetime.ctime()" does not
   invoke) conforms to the C standard.

datetime.strftime(format)

   Return a string representing the date and time, controlled by an
   explicit format string. For a complete list of formatting
   directives, see strftime() and strptime() Behavior.

datetime.__format__(format)

   Same as "datetime.strftime()". This makes it possible to specify a
   format string for a "datetime" object in formatted string literals
   and when using "str.format()". For a complete list of formatting
   directives, see strftime() and strptime() Behavior.


Examples of Usage: "datetime"
-----------------------------

Examples of working with "datetime" objects:

   >>> from datetime import datetime, date, time, timezone

   >>> # Using datetime.combine()
   >>> d = date(2005, 7, 14)
   >>> t = time(12, 30)
   >>> datetime.combine(d, t)
   datetime.datetime(2005, 7, 14, 12, 30)

   >>> # Using datetime.now()
   >>> datetime.now()   
   datetime.datetime(2007, 12, 6, 16, 29, 43, 79043)   # GMT +1
   >>> datetime.now(timezone.utc)   
   datetime.datetime(2007, 12, 6, 15, 29, 43, 79060, tzinfo=datetime.timezone.utc)

   >>> # Using datetime.strptime()
   >>> dt = datetime.strptime("21/11/06 16:30", "%d/%m/%y %H:%M")
   >>> dt
   datetime.datetime(2006, 11, 21, 16, 30)

   >>> # Using datetime.timetuple() to get tuple of all attributes
   >>> tt = dt.timetuple()
   >>> for it in tt:   
   ...     print(it)
   ...
   2006    # year
   11      # month
   21      # day
   16      # hour
   30      # minute
   0       # second
   1       # weekday (0 = Monday)
   325     # number of days since 1st January
   -1      # dst - method tzinfo.dst() returned None

   >>> # Date in ISO format
   >>> ic = dt.isocalendar()
   >>> for it in ic:   
   ...     print(it)
   ...
   2006    # ISO year
   47      # ISO week
   2       # ISO weekday

   >>> # Formatting a datetime
   >>> dt.strftime("%A, %d. %B %Y %I:%M%p")
   'Tuesday, 21. November 2006 04:30PM'
   >>> 'The {1} is {0:%d}, the {2} is {0:%B}, the {3} is {0:%I:%M%p}.'.format(dt, "day", "month", "time")
   'The day is 21, the month is November, the time is 04:30PM.'

The example below defines a "tzinfo" subclass capturing time zone
information for Kabul, Afghanistan, which used +4 UTC until 1945 and
then +4:30 UTC thereafter:

   from datetime import timedelta, datetime, tzinfo, timezone

   class KabulTz(tzinfo):
       # Kabul used +4 until 1945, when they moved to +4:30
       UTC_MOVE_DATE = datetime(1944, 12, 31, 20, tzinfo=timezone.utc)

       def utcoffset(self, dt):
           if dt.year < 1945:
               return timedelta(hours=4)
           elif (1945, 1, 1, 0, 0) <= dt.timetuple()[:5] < (1945, 1, 1, 0, 30):
               # An ambiguous ("imaginary") half-hour range representing
               # a 'fold' in time due to the shift from +4 to +4:30.
               # If dt falls in the imaginary range, use fold to decide how
               # to resolve. See PEP495.
               return timedelta(hours=4, minutes=(30 if dt.fold else 0))
           else:
               return timedelta(hours=4, minutes=30)

       def fromutc(self, dt):
           # Follow same validations as in datetime.tzinfo
           if not isinstance(dt, datetime):
               raise TypeError("fromutc() requires a datetime argument")
           if dt.tzinfo is not self:
               raise ValueError("dt.tzinfo is not self")

           # A custom implementation is required for fromutc as
           # the input to this function is a datetime with utc values
           # but with a tzinfo set to self.
           # See datetime.astimezone or fromtimestamp.
           if dt.replace(tzinfo=timezone.utc) >= self.UTC_MOVE_DATE:
               return dt + timedelta(hours=4, minutes=30)
           else:
               return dt + timedelta(hours=4)

       def dst(self, dt):
           # Kabul does not observe daylight saving time.
           return timedelta(0)

       def tzname(self, dt):
           if dt >= self.UTC_MOVE_DATE:
               return "+04:30"
           return "+04"

Usage of "KabulTz" from above:

   >>> tz1 = KabulTz()

   >>> # Datetime before the change
   >>> dt1 = datetime(1900, 11, 21, 16, 30, tzinfo=tz1)
   >>> print(dt1.utcoffset())
   4:00:00

   >>> # Datetime after the change
   >>> dt2 = datetime(2006, 6, 14, 13, 0, tzinfo=tz1)
   >>> print(dt2.utcoffset())
   4:30:00

   >>> # Convert datetime to another time zone
   >>> dt3 = dt2.astimezone(timezone.utc)
   >>> dt3
   datetime.datetime(2006, 6, 14, 8, 30, tzinfo=datetime.timezone.utc)
   >>> dt2
   datetime.datetime(2006, 6, 14, 13, 0, tzinfo=KabulTz())
   >>> dt2 == dt3
   True


"time" Objects
==============

A "time" object represents a (local) time of day, independent of any
particular day, and subject to adjustment via a "tzinfo" object.

class datetime.time(hour=0, minute=0, second=0, microsecond=0, tzinfo=None, *, fold=0)

   All arguments are optional. *tzinfo* may be "None", or an instance
   of a "tzinfo" subclass. The remaining arguments must be integers in
   the following ranges:

   * "0 <= hour < 24",

   * "0 <= minute < 60",

   * "0 <= second < 60",

   * "0 <= microsecond < 1000000",

   * "fold in [0, 1]".

   If an argument outside those ranges is given, "ValueError" is
   raised. All default to "0" except *tzinfo*, which defaults to
   "None".

Class attributes:

time.min

   The earliest representable "time", "time(0, 0, 0, 0)".

time.max

   The latest representable "time", "time(23, 59, 59, 999999)".

time.resolution

   The smallest possible difference between non-equal "time" objects,
   "timedelta(microseconds=1)", although note that arithmetic on
   "time" objects is not supported.

Instance attributes (read-only):

time.hour

   In "range(24)".

time.minute

   In "range(60)".

time.second

   In "range(60)".

time.microsecond

   In "range(1000000)".

time.tzinfo

   The object passed as the tzinfo argument to the "time" constructor,
   or "None" if none was passed.

time.fold

   In "[0, 1]". Used to disambiguate wall times during a repeated
   interval. (A repeated interval occurs when clocks are rolled back
   at the end of daylight saving time or when the UTC offset for the
   current zone is decreased for political reasons.) The value 0 (1)
   represents the earlier (later) of the two moments with the same
   wall time representation.

   New in version 3.6.

"time" objects support comparison of "time" to "time", where *a* is
considered less than *b* when *a* precedes *b* in time. If one
comparand is naive and the other is aware, "TypeError" is raised if an
order comparison is attempted. For equality comparisons, naive
instances are never equal to aware instances.

If both comparands are aware, and have the same "tzinfo" attribute,
the common "tzinfo" attribute is ignored and the base times are
compared. If both comparands are aware and have different "tzinfo"
attributes, the comparands are first adjusted by subtracting their UTC
offsets (obtained from "self.utcoffset()"). In order to stop mixed-
type comparisons from falling back to the default comparison by object
address, when a "time" object is compared to an object of a different
type, "TypeError" is raised unless the comparison is "==" or "!=". The
latter cases return "False" or "True", respectively.

Changed in version 3.3: Equality comparisons between aware and naive
"time" instances don't raise "TypeError".

In Boolean contexts, a "time" object is always considered to be true.

Changed in version 3.5: Before Python 3.5, a "time" object was
considered to be false if it represented midnight in UTC. This
behavior was considered obscure and error-prone and has been removed
in Python 3.5. See bpo-13936 for full details.

Other constructor:

classmethod time.fromisoformat(time_string)

   Return a "time" corresponding to a *time_string* in one of the
   formats emitted by "time.isoformat()". Specifically, this function
   supports strings in the format:

      HH[:MM[:SS[.fff[fff]]]][+HH:MM[:SS[.ffffff]]]

   Caution:

     This does *not* support parsing arbitrary ISO 8601 strings. It is
     only intended as the inverse operation of "time.isoformat()".

   Examples:

      >>> from datetime import time
      >>> time.fromisoformat('04:23:01')
      datetime.time(4, 23, 1)
      >>> time.fromisoformat('04:23:01.000384')
      datetime.time(4, 23, 1, 384)
      >>> time.fromisoformat('04:23:01+04:00')
      datetime.time(4, 23, 1, tzinfo=datetime.timezone(datetime.timedelta(seconds=14400)))

   New in version 3.7.

Instance methods:

time.replace(hour=self.hour, minute=self.minute, second=self.second, microsecond=self.microsecond, tzinfo=self.tzinfo, *, fold=0)

   Return a "time" with the same value, except for those attributes
   given new values by whichever keyword arguments are specified. Note
   that "tzinfo=None" can be specified to create a naive "time" from
   an aware "time", without conversion of the time data.

   New in version 3.6: Added the "fold" argument.

time.isoformat(timespec='auto')

   Return a string representing the time in ISO 8601 format, one of:

   * "HH:MM:SS.ffffff", if "microsecond" is not 0

   * "HH:MM:SS", if "microsecond" is 0

   * "HH:MM:SS.ffffff+HH:MM[:SS[.ffffff]]", if "utcoffset()" does not
     return "None"

   * "HH:MM:SS+HH:MM[:SS[.ffffff]]", if "microsecond" is 0 and
     "utcoffset()" does not return "None"

   The optional argument *timespec* specifies the number of additional
   components of the time to include (the default is "'auto'"). It can
   be one of the following:

   * "'auto'": Same as "'seconds'" if "microsecond" is 0, same as
     "'microseconds'" otherwise.

   * "'hours'": Include the "hour" in the two-digit "HH" format.

   * "'minutes'": Include "hour" and "minute" in "HH:MM" format.

   * "'seconds'": Include "hour", "minute", and "second" in "HH:MM:SS"
     format.

   * "'milliseconds'": Include full time, but truncate fractional
     second part to milliseconds. "HH:MM:SS.sss" format.

   * "'microseconds'": Include full time in "HH:MM:SS.ffffff" format.

   Note:

     Excluded time components are truncated, not rounded.

   "ValueError" will be raised on an invalid *timespec* argument.

   Example:

      >>> from datetime import time
      >>> time(hour=12, minute=34, second=56, microsecond=123456).isoformat(timespec='minutes')
      '12:34'
      >>> dt = time(hour=12, minute=34, second=56, microsecond=0)
      >>> dt.isoformat(timespec='microseconds')
      '12:34:56.000000'
      >>> dt.isoformat(timespec='auto')
      '12:34:56'

   New in version 3.6: Added the *timespec* argument.

time.__str__()

   For a time *t*, "str(t)" is equivalent to "t.isoformat()".

time.strftime(format)

   Return a string representing the time, controlled by an explicit
   format string. For a complete list of formatting directives, see
   strftime() and strptime() Behavior.

time.__format__(format)

   Same as "time.strftime()". This makes it possible to specify a
   format string for a "time" object in formatted string literals and
   when using "str.format()". For a complete list of formatting
   directives, see strftime() and strptime() Behavior.

time.utcoffset()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.utcoffset(None)", and raises an exception if the
   latter doesn't return "None" or a "timedelta" object with magnitude
   less than one day.

   Changed in version 3.7: The UTC offset is not restricted to a whole
   number of minutes.

time.dst()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.dst(None)", and raises an exception if the latter
   doesn't return "None", or a "timedelta" object with magnitude less
   than one day.

   Changed in version 3.7: The DST offset is not restricted to a whole
   number of minutes.

time.tzname()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.tzname(None)", or raises an exception if the latter
   doesn't return "None" or a string object.


Examples of Usage: "time"
-------------------------

Examples of working with a "time" object:

   >>> from datetime import time, tzinfo, timedelta
   >>> class TZ1(tzinfo):
   ...     def utcoffset(self, dt):
   ...         return timedelta(hours=1)
   ...     def dst(self, dt):
   ...         return timedelta(0)
   ...     def tzname(self,dt):
   ...         return "+01:00"
   ...     def  __repr__(self):
   ...         return f"{self.__class__.__name__}()"
   ...
   >>> t = time(12, 10, 30, tzinfo=TZ1())
   >>> t
   datetime.time(12, 10, 30, tzinfo=TZ1())
   >>> t.isoformat()
   '12:10:30+01:00'
   >>> t.dst()
   datetime.timedelta(0)
   >>> t.tzname()
   '+01:00'
   >>> t.strftime("%H:%M:%S %Z")
   '12:10:30 +01:00'
   >>> 'The {} is {:%H:%M}.'.format("time", t)
   'The time is 12:10.'


"tzinfo" Objects
================

class datetime.tzinfo

   This is an abstract base class, meaning that this class should not
   be instantiated directly.  Define a subclass of "tzinfo" to capture
   information about a particular time zone.

   An instance of (a concrete subclass of) "tzinfo" can be passed to
   the constructors for "datetime" and "time" objects. The latter
   objects view their attributes as being in local time, and the
   "tzinfo" object supports methods revealing offset of local time
   from UTC, the name of the time zone, and DST offset, all relative
   to a date or time object passed to them.

   You need to derive a concrete subclass, and (at least) supply
   implementations of the standard "tzinfo" methods needed by the
   "datetime" methods you use. The "datetime" module provides
   "timezone", a simple concrete subclass of "tzinfo" which can
   represent timezones with fixed offset from UTC such as UTC itself
   or North American EST and EDT.

   Special requirement for pickling:  A "tzinfo" subclass must have an
   "__init__()" method that can be called with no arguments, otherwise
   it can be pickled but possibly not unpickled again. This is a
   technical requirement that may be relaxed in the future.

   A concrete subclass of "tzinfo" may need to implement the following
   methods. Exactly which methods are needed depends on the uses made
   of aware "datetime" objects. If in doubt, simply implement all of
   them.

tzinfo.utcoffset(dt)

   Return offset of local time from UTC, as a "timedelta" object that
   is positive east of UTC. If local time is west of UTC, this should
   be negative.

   This represents the *total* offset from UTC; for example, if a
   "tzinfo" object represents both time zone and DST adjustments,
   "utcoffset()" should return their sum. If the UTC offset isn't
   known, return "None". Else the value returned must be a "timedelta"
   object strictly between "-timedelta(hours=24)" and
   "timedelta(hours=24)" (the magnitude of the offset must be less
   than one day). Most implementations of "utcoffset()" will probably
   look like one of these two:

      return CONSTANT                 # fixed-offset class
      return CONSTANT + self.dst(dt)  # daylight-aware class

   If "utcoffset()" does not return "None", "dst()" should not return
   "None" either.

   The default implementation of "utcoffset()" raises
   "NotImplementedError".

   Changed in version 3.7: The UTC offset is not restricted to a whole
   number of minutes.

tzinfo.dst(dt)

   Return the daylight saving time (DST) adjustment, as a "timedelta"
   object or "None" if DST information isn't known.

   Return "timedelta(0)" if DST is not in effect. If DST is in effect,
   return the offset as a "timedelta" object (see "utcoffset()" for
   details). Note that DST offset, if applicable, has already been
   added to the UTC offset returned by "utcoffset()", so there's no
   need to consult "dst()" unless you're interested in obtaining DST
   info separately. For example, "datetime.timetuple()" calls its
   "tzinfo" attribute's "dst()" method to determine how the "tm_isdst"
   flag should be set, and "tzinfo.fromutc()" calls "dst()" to account
   for DST changes when crossing time zones.

   An instance *tz* of a "tzinfo" subclass that models both standard
   and daylight times must be consistent in this sense:

   "tz.utcoffset(dt) - tz.dst(dt)"

   must return the same result for every "datetime" *dt* with
   "dt.tzinfo == tz"  For sane "tzinfo" subclasses, this expression
   yields the time zone's "standard offset", which should not depend
   on the date or the time, but only on geographic location. The
   implementation of "datetime.astimezone()" relies on this, but
   cannot detect violations; it's the programmer's responsibility to
   ensure it. If a "tzinfo" subclass cannot guarantee this, it may be
   able to override the default implementation of "tzinfo.fromutc()"
   to work correctly with "astimezone()" regardless.

   Most implementations of "dst()" will probably look like one of
   these two:

      def dst(self, dt):
          # a fixed-offset class:  doesn't account for DST
          return timedelta(0)

   or:

      def dst(self, dt):
          # Code to set dston and dstoff to the time zone's DST
          # transition times based on the input dt.year, and expressed
          # in standard local time.

          if dston <= dt.replace(tzinfo=None) < dstoff:
              return timedelta(hours=1)
          else:
              return timedelta(0)

   The default implementation of "dst()" raises "NotImplementedError".

   Changed in version 3.7: The DST offset is not restricted to a whole
   number of minutes.

tzinfo.tzname(dt)

   Return the time zone name corresponding to the "datetime" object
   *dt*, as a string. Nothing about string names is defined by the
   "datetime" module, and there's no requirement that it mean anything
   in particular. For example, "GMT", "UTC", "-500", "-5:00", "EDT",
   "US/Eastern", "America/New York" are all valid replies. Return
   "None" if a string name isn't known. Note that this is a method
   rather than a fixed string primarily because some "tzinfo"
   subclasses will wish to return different names depending on the
   specific value of *dt* passed, especially if the "tzinfo" class is
   accounting for daylight time.

   The default implementation of "tzname()" raises
   "NotImplementedError".

These methods are called by a "datetime" or "time" object, in response
to their methods of the same names. A "datetime" object passes itself
as the argument, and a "time" object passes "None" as the argument. A
"tzinfo" subclass's methods should therefore be prepared to accept a
*dt* argument of "None", or of class "datetime".

When "None" is passed, it's up to the class designer to decide the
best response. For example, returning "None" is appropriate if the
class wishes to say that time objects don't participate in the
"tzinfo" protocols. It may be more useful for "utcoffset(None)" to
return the standard UTC offset, as there is no other convention for
discovering the standard offset.

When a "datetime" object is passed in response to a "datetime" method,
"dt.tzinfo" is the same object as *self*. "tzinfo" methods can rely on
this, unless user code calls "tzinfo" methods directly. The intent is
that the "tzinfo" methods interpret *dt* as being in local time, and
not need worry about objects in other timezones.

There is one more "tzinfo" method that a subclass may wish to
override:

tzinfo.fromutc(dt)

   This is called from the default "datetime.astimezone()"
   implementation. When called from that, "dt.tzinfo" is *self*, and
   *dt*'s date and time data are to be viewed as expressing a UTC
   time. The purpose of "fromutc()" is to adjust the date and time
   data, returning an equivalent datetime in *self*'s local time.

   Most "tzinfo" subclasses should be able to inherit the default
   "fromutc()" implementation without problems. It's strong enough to
   handle fixed-offset time zones, and time zones accounting for both
   standard and daylight time, and the latter even if the DST
   transition times differ in different years. An example of a time
   zone the default "fromutc()" implementation may not handle
   correctly in all cases is one where the standard offset (from UTC)
   depends on the specific date and time passed, which can happen for
   political reasons. The default implementations of "astimezone()"
   and "fromutc()" may not produce the result you want if the result
   is one of the hours straddling the moment the standard offset
   changes.

   Skipping code for error cases, the default "fromutc()"
   implementation acts like:

      def fromutc(self, dt):
          # raise ValueError error if dt.tzinfo is not self
          dtoff = dt.utcoffset()
          dtdst = dt.dst()
          # raise ValueError if dtoff is None or dtdst is None
          delta = dtoff - dtdst  # this is self's standard offset
          if delta:
              dt += delta   # convert to standard local time
              dtdst = dt.dst()
              # raise ValueError if dtdst is None
          if dtdst:
              return dt + dtdst
          else:
              return dt

In the following "tzinfo_examples.py" file there are some examples of
"tzinfo" classes:

   from datetime import tzinfo, timedelta, datetime

   ZERO = timedelta(0)
   HOUR = timedelta(hours=1)
   SECOND = timedelta(seconds=1)

   # A class capturing the platform's idea of local time.
   # (May result in wrong values on historical times in
   #  timezones where UTC offset and/or the DST rules had
   #  changed in the past.)
   import time as _time

   STDOFFSET = timedelta(seconds = -_time.timezone)
   if _time.daylight:
       DSTOFFSET = timedelta(seconds = -_time.altzone)
   else:
       DSTOFFSET = STDOFFSET

   DSTDIFF = DSTOFFSET - STDOFFSET

   class LocalTimezone(tzinfo):

       def fromutc(self, dt):
           assert dt.tzinfo is self
           stamp = (dt - datetime(1970, 1, 1, tzinfo=self)) // SECOND
           args = _time.localtime(stamp)[:6]
           dst_diff = DSTDIFF // SECOND
           # Detect fold
           fold = (args == _time.localtime(stamp - dst_diff))
           return datetime(*args, microsecond=dt.microsecond,
                           tzinfo=self, fold=fold)

       def utcoffset(self, dt):
           if self._isdst(dt):
               return DSTOFFSET
           else:
               return STDOFFSET

       def dst(self, dt):
           if self._isdst(dt):
               return DSTDIFF
           else:
               return ZERO

       def tzname(self, dt):
           return _time.tzname[self._isdst(dt)]

       def _isdst(self, dt):
           tt = (dt.year, dt.month, dt.day,
                 dt.hour, dt.minute, dt.second,
                 dt.weekday(), 0, 0)
           stamp = _time.mktime(tt)
           tt = _time.localtime(stamp)
           return tt.tm_isdst > 0

   Local = LocalTimezone()


   # A complete implementation of current DST rules for major US time zones.

   def first_sunday_on_or_after(dt):
       days_to_go = 6 - dt.weekday()
       if days_to_go:
           dt += timedelta(days_to_go)
       return dt


   # US DST Rules
   #
   # This is a simplified (i.e., wrong for a few cases) set of rules for US
   # DST start and end times. For a complete and up-to-date set of DST rules
   # and timezone definitions, visit the Olson Database (or try pytz):
   # http://www.twinsun.com/tz/tz-link.htm
   # http://sourceforge.net/projects/pytz/ (might not be up-to-date)
   #
   # In the US, since 2007, DST starts at 2am (standard time) on the second
   # Sunday in March, which is the first Sunday on or after Mar 8.
   DSTSTART_2007 = datetime(1, 3, 8, 2)
   # and ends at 2am (DST time) on the first Sunday of Nov.
   DSTEND_2007 = datetime(1, 11, 1, 2)
   # From 1987 to 2006, DST used to start at 2am (standard time) on the first
   # Sunday in April and to end at 2am (DST time) on the last
   # Sunday of October, which is the first Sunday on or after Oct 25.
   DSTSTART_1987_2006 = datetime(1, 4, 1, 2)
   DSTEND_1987_2006 = datetime(1, 10, 25, 2)
   # From 1967 to 1986, DST used to start at 2am (standard time) on the last
   # Sunday in April (the one on or after April 24) and to end at 2am (DST time)
   # on the last Sunday of October, which is the first Sunday
   # on or after Oct 25.
   DSTSTART_1967_1986 = datetime(1, 4, 24, 2)
   DSTEND_1967_1986 = DSTEND_1987_2006

   def us_dst_range(year):
       # Find start and end times for US DST. For years before 1967, return
       # start = end for no DST.
       if 2006 < year:
           dststart, dstend = DSTSTART_2007, DSTEND_2007
       elif 1986 < year < 2007:
           dststart, dstend = DSTSTART_1987_2006, DSTEND_1987_2006
       elif 1966 < year < 1987:
           dststart, dstend = DSTSTART_1967_1986, DSTEND_1967_1986
       else:
           return (datetime(year, 1, 1), ) * 2

       start = first_sunday_on_or_after(dststart.replace(year=year))
       end = first_sunday_on_or_after(dstend.replace(year=year))
       return start, end


   class USTimeZone(tzinfo):

       def __init__(self, hours, reprname, stdname, dstname):
           self.stdoffset = timedelta(hours=hours)
           self.reprname = reprname
           self.stdname = stdname
           self.dstname = dstname

       def __repr__(self):
           return self.reprname

       def tzname(self, dt):
           if self.dst(dt):
               return self.dstname
           else:
               return self.stdname

       def utcoffset(self, dt):
           return self.stdoffset + self.dst(dt)

       def dst(self, dt):
           if dt is None or dt.tzinfo is None:
               # An exception may be sensible here, in one or both cases.
               # It depends on how you want to treat them.  The default
               # fromutc() implementation (called by the default astimezone()
               # implementation) passes a datetime with dt.tzinfo is self.
               return ZERO
           assert dt.tzinfo is self
           start, end = us_dst_range(dt.year)
           # Can't compare naive to aware objects, so strip the timezone from
           # dt first.
           dt = dt.replace(tzinfo=None)
           if start + HOUR <= dt < end - HOUR:
               # DST is in effect.
               return HOUR
           if end - HOUR <= dt < end:
               # Fold (an ambiguous hour): use dt.fold to disambiguate.
               return ZERO if dt.fold else HOUR
           if start <= dt < start + HOUR:
               # Gap (a non-existent hour): reverse the fold rule.
               return HOUR if dt.fold else ZERO
           # DST is off.
           return ZERO

       def fromutc(self, dt):
           assert dt.tzinfo is self
           start, end = us_dst_range(dt.year)
           start = start.replace(tzinfo=self)
           end = end.replace(tzinfo=self)
           std_time = dt + self.stdoffset
           dst_time = std_time + HOUR
           if end <= dst_time < end + HOUR:
               # Repeated hour
               return std_time.replace(fold=1)
           if std_time < start or dst_time >= end:
               # Standard time
               return std_time
           if start <= std_time < end - HOUR:
               # Daylight saving time
               return dst_time


   Eastern  = USTimeZone(-5, "Eastern",  "EST", "EDT")
   Central  = USTimeZone(-6, "Central",  "CST", "CDT")
   Mountain = USTimeZone(-7, "Mountain", "MST", "MDT")
   Pacific  = USTimeZone(-8, "Pacific",  "PST", "PDT")

Note that there are unavoidable subtleties twice per year in a
"tzinfo" subclass accounting for both standard and daylight time, at
the DST transition points. For concreteness, consider US Eastern (UTC
-0500), where EDT begins the minute after 1:59 (EST) on the second
Sunday in March, and ends the minute after 1:59 (EDT) on the first
Sunday in November:

     UTC   3:MM  4:MM  5:MM  6:MM  7:MM  8:MM
     EST  22:MM 23:MM  0:MM  1:MM  2:MM  3:MM
     EDT  23:MM  0:MM  1:MM  2:MM  3:MM  4:MM

   start  22:MM 23:MM  0:MM  1:MM  3:MM  4:MM

     end  23:MM  0:MM  1:MM  1:MM  2:MM  3:MM

When DST starts (the "start" line), the local wall clock leaps from
1:59 to 3:00. A wall time of the form 2:MM doesn't really make sense
on that day, so "astimezone(Eastern)" won't deliver a result with
"hour == 2" on the day DST begins. For example, at the Spring forward
transition of 2016, we get:

   >>> from datetime import datetime, timezone
   >>> from tzinfo_examples import HOUR, Eastern
   >>> u0 = datetime(2016, 3, 13, 5, tzinfo=timezone.utc)
   >>> for i in range(4):
   ...     u = u0 + i*HOUR
   ...     t = u.astimezone(Eastern)
   ...     print(u.time(), 'UTC =', t.time(), t.tzname())
   ...
   05:00:00 UTC = 00:00:00 EST
   06:00:00 UTC = 01:00:00 EST
   07:00:00 UTC = 03:00:00 EDT
   08:00:00 UTC = 04:00:00 EDT

When DST ends (the "end" line), there's a potentially worse problem:
there's an hour that can't be spelled unambiguously in local wall
time: the last hour of daylight time. In Eastern, that's times of the
form 5:MM UTC on the day daylight time ends. The local wall clock
leaps from 1:59 (daylight time) back to 1:00 (standard time) again.
Local times of the form 1:MM are ambiguous. "astimezone()" mimics the
local clock's behavior by mapping two adjacent UTC hours into the same
local hour then. In the Eastern example, UTC times of the form 5:MM
and 6:MM both map to 1:MM when converted to Eastern, but earlier times
have the "fold" attribute set to 0 and the later times have it set to
1. For example, at the Fall back transition of 2016, we get:

   >>> u0 = datetime(2016, 11, 6, 4, tzinfo=timezone.utc)
   >>> for i in range(4):
   ...     u = u0 + i*HOUR
   ...     t = u.astimezone(Eastern)
   ...     print(u.time(), 'UTC =', t.time(), t.tzname(), t.fold)
   ...
   04:00:00 UTC = 00:00:00 EDT 0
   05:00:00 UTC = 01:00:00 EDT 0
   06:00:00 UTC = 01:00:00 EST 1
   07:00:00 UTC = 02:00:00 EST 0

Note that the "datetime" instances that differ only by the value of
the "fold" attribute are considered equal in comparisons.

Applications that can't bear wall-time ambiguities should explicitly
check the value of the "fold" attribute or avoid using hybrid "tzinfo"
subclasses; there are no ambiguities when using "timezone", or any
other fixed-offset "tzinfo" subclass (such as a class representing
only EST (fixed offset -5 hours), or only EDT (fixed offset -4
hours)).

See also:

  dateutil.tz
     The "datetime" module has a basic "timezone" class (for handling
     arbitrary fixed offsets from UTC) and its "timezone.utc"
     attribute (a UTC timezone instance).

     *dateutil.tz* library brings the *IANA timezone database* (also
     known as the Olson database) to Python, and its usage is
     recommended.

  IANA timezone database
     The Time Zone Database (often called tz, tzdata or zoneinfo)
     contains code and data that represent the history of local time
     for many representative locations around the globe. It is updated
     periodically to reflect changes made by political bodies to time
     zone boundaries, UTC offsets, and daylight-saving rules.


"timezone" Objects
==================

The "timezone" class is a subclass of "tzinfo", each instance of which
represents a timezone defined by a fixed offset from UTC.

Objects of this class cannot be used to represent timezone information
in the locations where different offsets are used in different days of
the year or where historical changes have been made to civil time.

class datetime.timezone(offset, name=None)

   The *offset* argument must be specified as a "timedelta" object
   representing the difference between the local time and UTC. It must
   be strictly between "-timedelta(hours=24)" and
   "timedelta(hours=24)", otherwise "ValueError" is raised.

   The *name* argument is optional. If specified it must be a string
   that will be used as the value returned by the "datetime.tzname()"
   method.

   New in version 3.2.

   Changed in version 3.7: The UTC offset is not restricted to a whole
   number of minutes.

timezone.utcoffset(dt)

   Return the fixed value specified when the "timezone" instance is
   constructed.

   The *dt* argument is ignored. The return value is a "timedelta"
   instance equal to the difference between the local time and UTC.

   Changed in version 3.7: The UTC offset is not restricted to a whole
   number of minutes.

timezone.tzname(dt)

   Return the fixed value specified when the "timezone" instance is
   constructed.

   If *name* is not provided in the constructor, the name returned by
   "tzname(dt)" is generated from the value of the "offset" as
   follows. If *offset* is "timedelta(0)", the name is "UTC",
   otherwise it is a string in the format "UTC±HH:MM", where ± is the
   sign of "offset", HH and MM are two digits of "offset.hours" and
   "offset.minutes" respectively.

   Changed in version 3.6: Name generated from "offset=timedelta(0)"
   is now plain *'UTC'*, not "'UTC+00:00'".

timezone.dst(dt)

   Always returns "None".

timezone.fromutc(dt)

   Return "dt + offset". The *dt* argument must be an aware "datetime"
   instance, with "tzinfo" set to "self".

Class attributes:

timezone.utc

   The UTC timezone, "timezone(timedelta(0))".


"strftime()" and "strptime()" Behavior
======================================

"date", "datetime", and "time" objects all support a
"strftime(format)" method, to create a string representing the time
under the control of an explicit format string.

Conversely, the "datetime.strptime()" class method creates a
"datetime" object from a string representing a date and time and a
corresponding format string.

The table below provides a high-level comparison of "strftime()"
versus "strptime()":

+------------------+----------------------------------------------------------+--------------------------------------------------------------------------------+
|                  | "strftime"                                               | "strptime"                                                                     |
|==================|==========================================================|================================================================================|
| Usage            | Convert object to a string according to a given format   | Parse a string into a "datetime" object given a corresponding format           |
+------------------+----------------------------------------------------------+--------------------------------------------------------------------------------+
| Type of method   | Instance method                                          | Class method                                                                   |
+------------------+----------------------------------------------------------+--------------------------------------------------------------------------------+
| Method of        | "date"; "datetime"; "time"                               | "datetime"                                                                     |
+------------------+----------------------------------------------------------+--------------------------------------------------------------------------------+
| Signature        | "strftime(format)"                                       | "strptime(date_string, format)"                                                |
+------------------+----------------------------------------------------------+--------------------------------------------------------------------------------+


"strftime()" and "strptime()" Format Codes
------------------------------------------

The following is a list of all the format codes that the 1989 C
standard requires, and these work on all platforms with a standard C
implementation.

+-------------+----------------------------------+--------------------------+---------+
| Directive   | Meaning                          | Example                  | Notes   |
|=============|==================================|==========================|=========|
| "%a"        | Weekday as locale's abbreviated  | Sun, Mon, ..., Sat       | (1)     |
|             | name.                            | (en_US); So, Mo, ..., Sa |         |
|             |                                  | (de_DE)                  |         |
+-------------+----------------------------------+--------------------------+---------+
| "%A"        | Weekday as locale's full name.   | Sunday, Monday, ...,     | (1)     |
|             |                                  | Saturday (en_US);        |         |
|             |                                  | Sonntag, Montag, ...,    |         |
|             |                                  | Samstag (de_DE)          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%w"        | Weekday as a decimal number,     | 0, 1, ..., 6             |         |
|             | where 0 is Sunday and 6 is       |                          |         |
|             | Saturday.                        |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%d"        | Day of the month as a zero-      | 01, 02, ..., 31          | (9)     |
|             | padded decimal number.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%b"        | Month as locale's abbreviated    | Jan, Feb, ..., Dec       | (1)     |
|             | name.                            | (en_US); Jan, Feb, ...,  |         |
|             |                                  | Dez (de_DE)              |         |
+-------------+----------------------------------+--------------------------+---------+
| "%B"        | Month as locale's full name.     | January, February, ...,  | (1)     |
|             |                                  | December (en_US);        |         |
|             |                                  | Januar, Februar, ...,    |         |
|             |                                  | Dezember (de_DE)         |         |
+-------------+----------------------------------+--------------------------+---------+
| "%m"        | Month as a zero-padded decimal   | 01, 02, ..., 12          | (9)     |
|             | number.                          |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%y"        | Year without century as a zero-  | 00, 01, ..., 99          | (9)     |
|             | padded decimal number.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%Y"        | Year with century as a decimal   | 0001, 0002, ..., 2013,   | (2)     |
|             | number.                          | 2014, ..., 9998, 9999    |         |
+-------------+----------------------------------+--------------------------+---------+
| "%H"        | Hour (24-hour clock) as a zero-  | 00, 01, ..., 23          | (9)     |
|             | padded decimal number.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%I"        | Hour (12-hour clock) as a zero-  | 01, 02, ..., 12          | (9)     |
|             | padded decimal number.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%p"        | Locale's equivalent of either AM | AM, PM (en_US); am, pm   | (1),    |
|             | or PM.                           | (de_DE)                  | (3)     |
+-------------+----------------------------------+--------------------------+---------+
| "%M"        | Minute as a zero-padded decimal  | 00, 01, ..., 59          | (9)     |
|             | number.                          |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%S"        | Second as a zero-padded decimal  | 00, 01, ..., 59          | (4),    |
|             | number.                          |                          | (9)     |
+-------------+----------------------------------+--------------------------+---------+
| "%f"        | Microsecond as a decimal number, | 000000, 000001, ...,     | (5)     |
|             | zero-padded on the left.         | 999999                   |         |
+-------------+----------------------------------+--------------------------+---------+
| "%z"        | UTC offset in the form           | (empty), +0000, -0400,   | (6)     |
|             | "±HHMM[SS[.ffffff]]" (empty      | +1030, +063415,          |         |
|             | string if the object is naive).  | -030712.345216           |         |
+-------------+----------------------------------+--------------------------+---------+
| "%Z"        | Time zone name (empty string if  | (empty), UTC, GMT        | (6)     |
|             | the object is naive).            |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%j"        | Day of the year as a zero-padded | 001, 002, ..., 366       | (9)     |
|             | decimal number.                  |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%U"        | Week number of the year (Sunday  | 00, 01, ..., 53          | (7),    |
|             | as the first day of the week) as |                          | (9)     |
|             | a zero padded decimal number.    |                          |         |
|             | All days in a new year preceding |                          |         |
|             | the first Sunday are considered  |                          |         |
|             | to be in week 0.                 |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%W"        | Week number of the year (Monday  | 00, 01, ..., 53          | (7),    |
|             | as the first day of the week) as |                          | (9)     |
|             | a decimal number. All days in a  |                          |         |
|             | new year preceding the first     |                          |         |
|             | Monday are considered to be in   |                          |         |
|             | week 0.                          |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%c"        | Locale's appropriate date and    | Tue Aug 16 21:30:00 1988 | (1)     |
|             | time representation.             | (en_US); Di 16 Aug       |         |
|             |                                  | 21:30:00 1988 (de_DE)    |         |
+-------------+----------------------------------+--------------------------+---------+
| "%x"        | Locale's appropriate date        | 08/16/88 (None);         | (1)     |
|             | representation.                  | 08/16/1988 (en_US);      |         |
|             |                                  | 16.08.1988 (de_DE)       |         |
+-------------+----------------------------------+--------------------------+---------+
| "%X"        | Locale's appropriate time        | 21:30:00 (en_US);        | (1)     |
|             | representation.                  | 21:30:00 (de_DE)         |         |
+-------------+----------------------------------+--------------------------+---------+
| "%%"        | A literal "'%'" character.       | %                        |         |
+-------------+----------------------------------+--------------------------+---------+

Several additional directives not required by the C89 standard are
included for convenience. These parameters all correspond to ISO 8601
date values.

+-------------+----------------------------------+--------------------------+---------+
| Directive   | Meaning                          | Example                  | Notes   |
|=============|==================================|==========================|=========|
| "%G"        | ISO 8601 year with century       | 0001, 0002, ..., 2013,   | (8)     |
|             | representing the year that       | 2014, ..., 9998, 9999    |         |
|             | contains the greater part of the |                          |         |
|             | ISO week ("%V").                 |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%u"        | ISO 8601 weekday as a decimal    | 1, 2, ..., 7             |         |
|             | number where 1 is Monday.        |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%V"        | ISO 8601 week as a decimal       | 01, 02, ..., 53          | (8),    |
|             | number with Monday as the first  |                          | (9)     |
|             | day of the week. Week 01 is the  |                          |         |
|             | week containing Jan 4.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+

These may not be available on all platforms when used with the
"strftime()" method. The ISO 8601 year and ISO 8601 week directives
are not interchangeable with the year and week number directives
above. Calling "strptime()" with incomplete or ambiguous ISO 8601
directives will raise a "ValueError".

The full set of format codes supported varies across platforms,
because Python calls the platform C library's "strftime()" function,
and platform variations are common. To see the full set of format
codes supported on your platform, consult the *strftime(3)*
documentation.

New in version 3.6: "%G", "%u" and "%V" were added.


Technical Detail
----------------

Broadly speaking, "d.strftime(fmt)" acts like the "time" module's
"time.strftime(fmt, d.timetuple())" although not all objects support a
"timetuple()" method.

For the "datetime.strptime()" class method, the default value is
"1900-01-01T00:00:00.000": any components not specified in the format
string will be pulled from the default value. [4]

Using "datetime.strptime(date_string, format)" is equivalent to:

   datetime(*(time.strptime(date_string, format)[0:6]))

except when the format includes sub-second components or timezone
offset information, which are supported in "datetime.strptime" but are
discarded by "time.strptime".

For "time" objects, the format codes for year, month, and day should
not be used, as "time" objects have no such values. If they're used
anyway, "1900" is substituted for the year, and "1" for the month and
day.

For "date" objects, the format codes for hours, minutes, seconds, and
microseconds should not be used, as "date" objects have no such
values. If they're used anyway, "0" is substituted for them.

For the same reason, handling of format strings containing Unicode
code points that can't be represented in the charset of the current
locale is also platform-dependent. On some platforms such code points
are preserved intact in the output, while on others "strftime" may
raise "UnicodeError" or return an empty string instead.

Notes:

1. Because the format depends on the current locale, care should be
   taken when making assumptions about the output value. Field
   orderings will vary (for example, "month/day/year" versus
   "day/month/year"), and the output may contain Unicode characters
   encoded using the locale's default encoding (for example, if the
   current locale is "ja_JP", the default encoding could be any one of
   "eucJP", "SJIS", or "utf-8"; use "locale.getlocale()" to determine
   the current locale's encoding).

2. The "strptime()" method can parse years in the full [1, 9999]
   range, but years < 1000 must be zero-filled to 4-digit width.

   Changed in version 3.2: In previous versions, "strftime()" method
   was restricted to years >= 1900.

   Changed in version 3.3: In version 3.2, "strftime()" method was
   restricted to years >= 1000.

3. When used with the "strptime()" method, the "%p" directive only
   affects the output hour field if the "%I" directive is used to
   parse the hour.

4. Unlike the "time" module, the "datetime" module does not support
   leap seconds.

5. When used with the "strptime()" method, the "%f" directive accepts
   from one to six digits and zero pads on the right. "%f" is an
   extension to the set of format characters in the C standard (but
   implemented separately in datetime objects, and therefore always
   available).

6. For a naive object, the "%z" and "%Z" format codes are replaced by
   empty strings.

   For an aware object:

   "%z"
      "utcoffset()" is transformed into a string of the form
      "±HHMM[SS[.ffffff]]", where "HH" is a 2-digit string giving the
      number of UTC offset hours, "MM" is a 2-digit string giving the
      number of UTC offset minutes, "SS" is a 2-digit string giving
      the number of UTC offset seconds and "ffffff" is a 6-digit
      string giving the number of UTC offset microseconds. The
      "ffffff" part is omitted when the offset is a whole number of
      seconds and both the "ffffff" and the "SS" part is omitted when
      the offset is a whole number of minutes. For example, if
      "utcoffset()" returns "timedelta(hours=-3, minutes=-30)", "%z"
      is replaced with the string "'-0330'".

   Changed in version 3.7: The UTC offset is not restricted to a whole
   number of minutes.

   Changed in version 3.7: When the "%z" directive is provided to the
   "strptime()" method, the UTC offsets can have a colon as a
   separator between hours, minutes and seconds. For example,
   "'+01:00:00'" will be parsed as an offset of one hour. In addition,
   providing "'Z'" is identical to "'+00:00'".

   "%Z"
      In "strftime()", "%Z" is replaced by an empty string if
      "tzname()" returns "None"; otherwise "%Z" is replaced by the
      returned value, which must be a string.

      "strptime()" only accepts certain values for "%Z":

      1. any value in "time.tzname" for your machine's locale

      2. the hard-coded values "UTC" and "GMT"

      So someone living in Japan may have "JST", "UTC", and "GMT" as
      valid values, but probably not "EST". It will raise "ValueError"
      for invalid values.

   Changed in version 3.2: When the "%z" directive is provided to the
   "strptime()" method, an aware "datetime" object will be produced.
   The "tzinfo" of the result will be set to a "timezone" instance.

7. When used with the "strptime()" method, "%U" and "%W" are only used
   in calculations when the day of the week and the calendar year
   ("%Y") are specified.

8. Similar to "%U" and "%W", "%V" is only used in calculations when
   the day of the week and the ISO year ("%G") are specified in a
   "strptime()" format string. Also note that "%G" and "%Y" are not
   interchangeable.

9. When used with the "strptime()" method, the leading zero is
   optional for  formats "%d", "%m", "%H", "%I", "%M", "%S", "%J",
   "%U", "%W", and "%V". Format "%y" does require a leading zero.

-[ Footnotes ]-

[1] If, that is, we ignore the effects of Relativity

[2] This matches the definition of the "proleptic Gregorian" calendar
    in Dershowitz and Reingold's book *Calendrical Calculations*,
    where it's the base calendar for all computations. See the book
    for algorithms for converting between proleptic Gregorian ordinals
    and many other calendar systems.

[3] See R. H. van Gent's guide to the mathematics of the ISO 8601
    calendar for a good explanation.

[4] Passing "datetime.strptime('Feb 29', '%b %d')" will fail since
    "1900" is not a leap year.
