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1:mod:`time` --- Time access and conversions
2===========================================
3
4.. module:: time
5   :synopsis: Time access and conversions.
6
7--------------
8
9This module provides various time-related functions. For related
10functionality, see also the :mod:`datetime` and :mod:`calendar` modules.
11
12Although this module is always available,
13not all functions are available on all platforms.  Most of the functions
14defined in this module call platform C library functions with the same name.  It
15may sometimes be helpful to consult the platform documentation, because the
16semantics of these functions varies among platforms.
17
18An explanation of some terminology and conventions is in order.
19
20.. _epoch:
21
22.. index:: single: epoch
23
24* The :dfn:`epoch` is the point where the time starts, and is platform
25  dependent.  For Unix, the epoch is January 1, 1970, 00:00:00 (UTC).
26  To find out what the epoch is on a given platform, look at
27  ``time.gmtime(0)``.
28
29.. _leap seconds: https://en.wikipedia.org/wiki/Leap_second
30
31.. index:: seconds since the epoch
32
33* The term :dfn:`seconds since the epoch` refers to the total number
34  of elapsed seconds since the epoch, typically excluding
35  `leap seconds`_.  Leap seconds are excluded from this total on all
36  POSIX-compliant platforms.
37
38.. index:: single: Year 2038
39
40* The functions in this module may not handle dates and times before the epoch or
41  far in the future.  The cut-off point in the future is determined by the C
42  library; for 32-bit systems, it is typically in 2038.
43
44.. index::
45   single: 2-digit years
46
47* Function :func:`strptime` can parse 2-digit years when given ``%y`` format
48  code. When 2-digit years are parsed, they are converted according to the POSIX
49  and ISO C standards: values 69--99 are mapped to 1969--1999, and values 0--68
50  are mapped to 2000--2068.
51
52.. index::
53   single: UTC
54   single: Coordinated Universal Time
55   single: Greenwich Mean Time
56
57* UTC is Coordinated Universal Time (formerly known as Greenwich Mean Time, or
58  GMT).  The acronym UTC is not a mistake but a compromise between English and
59  French.
60
61.. index:: single: Daylight Saving Time
62
63* DST is Daylight Saving Time, an adjustment of the timezone by (usually) one
64  hour during part of the year.  DST rules are magic (determined by local law) and
65  can change from year to year.  The C library has a table containing the local
66  rules (often it is read from a system file for flexibility) and is the only
67  source of True Wisdom in this respect.
68
69* The precision of the various real-time functions may be less than suggested by
70  the units in which their value or argument is expressed. E.g. on most Unix
71  systems, the clock "ticks" only 50 or 100 times a second.
72
73* On the other hand, the precision of :func:`.time` and :func:`sleep` is better
74  than their Unix equivalents: times are expressed as floating point numbers,
75  :func:`.time` returns the most accurate time available (using Unix
76  :c:func:`gettimeofday` where available), and :func:`sleep` will accept a time
77  with a nonzero fraction (Unix :c:func:`select` is used to implement this, where
78  available).
79
80* The time value as returned by :func:`gmtime`, :func:`localtime`, and
81  :func:`strptime`, and accepted by :func:`asctime`, :func:`mktime` and
82  :func:`strftime`, is a sequence of 9 integers.  The return values of
83  :func:`gmtime`, :func:`localtime`, and :func:`strptime` also offer attribute
84  names for individual fields.
85
86  See :class:`struct_time` for a description of these objects.
87
88  .. versionchanged:: 3.3
89     The :class:`struct_time` type was extended to provide the :attr:`tm_gmtoff`
90     and :attr:`tm_zone` attributes when platform supports corresponding
91     ``struct tm`` members.
92
93  .. versionchanged:: 3.6
94     The :class:`struct_time` attributes :attr:`tm_gmtoff` and :attr:`tm_zone`
95     are now available on all platforms.
96
97* Use the following functions to convert between time representations:
98
99  +-------------------------+-------------------------+-------------------------+
100  | From                    | To                      | Use                     |
101  +=========================+=========================+=========================+
102  | seconds since the epoch | :class:`struct_time` in | :func:`gmtime`          |
103  |                         | UTC                     |                         |
104  +-------------------------+-------------------------+-------------------------+
105  | seconds since the epoch | :class:`struct_time` in | :func:`localtime`       |
106  |                         | local time              |                         |
107  +-------------------------+-------------------------+-------------------------+
108  | :class:`struct_time` in | seconds since the epoch | :func:`calendar.timegm` |
109  | UTC                     |                         |                         |
110  +-------------------------+-------------------------+-------------------------+
111  | :class:`struct_time` in | seconds since the epoch | :func:`mktime`          |
112  | local time              |                         |                         |
113  +-------------------------+-------------------------+-------------------------+
114
115
116.. _time-functions:
117
118Functions
119---------
120
121.. function:: asctime([t])
122
123   Convert a tuple or :class:`struct_time` representing a time as returned by
124   :func:`gmtime` or :func:`localtime` to a string of the following
125   form: ``'Sun Jun 20 23:21:05 1993'``. The day field is two characters long
126   and is space padded if the day is a single digit,
127   e.g.: ``'Wed Jun  9 04:26:40 1993'``.
128
129   If *t* is not provided, the current time as returned by :func:`localtime`
130   is used. Locale information is not used by :func:`asctime`.
131
132   .. note::
133
134      Unlike the C function of the same name, :func:`asctime` does not add a
135      trailing newline.
136
137.. function:: pthread_getcpuclockid(thread_id)
138
139   Return the *clk_id* of the thread-specific CPU-time clock for the specified *thread_id*.
140
141   Use :func:`threading.get_ident` or the :attr:`~threading.Thread.ident`
142   attribute of :class:`threading.Thread` objects to get a suitable value
143   for *thread_id*.
144
145   .. warning::
146      Passing an invalid or expired *thread_id* may result in
147      undefined behavior, such as segmentation fault.
148
149   .. availability:: Unix (see the man page for :manpage:`pthread_getcpuclockid(3)` for
150      further information).
151
152   .. versionadded:: 3.7
153
154.. function:: clock_getres(clk_id)
155
156   Return the resolution (precision) of the specified clock *clk_id*.  Refer to
157   :ref:`time-clock-id-constants` for a list of accepted values for *clk_id*.
158
159   .. availability:: Unix.
160
161   .. versionadded:: 3.3
162
163
164.. function:: clock_gettime(clk_id) -> float
165
166   Return the time of the specified clock *clk_id*.  Refer to
167   :ref:`time-clock-id-constants` for a list of accepted values for *clk_id*.
168
169   .. availability:: Unix.
170
171   .. versionadded:: 3.3
172
173
174.. function:: clock_gettime_ns(clk_id) -> int
175
176   Similar to :func:`clock_gettime` but return time as nanoseconds.
177
178   .. availability:: Unix.
179
180   .. versionadded:: 3.7
181
182
183.. function:: clock_settime(clk_id, time: float)
184
185   Set the time of the specified clock *clk_id*.  Currently,
186   :data:`CLOCK_REALTIME` is the only accepted value for *clk_id*.
187
188   .. availability:: Unix.
189
190   .. versionadded:: 3.3
191
192
193.. function:: clock_settime_ns(clk_id, time: int)
194
195   Similar to :func:`clock_settime` but set time with nanoseconds.
196
197   .. availability:: Unix.
198
199   .. versionadded:: 3.7
200
201
202.. function:: ctime([secs])
203
204   Convert a time expressed in seconds since the epoch to a string of a form:
205   ``'Sun Jun 20 23:21:05 1993'`` representing local time. The day field
206   is two characters long and is space padded if the day is a single digit,
207   e.g.: ``'Wed Jun  9 04:26:40 1993'``.
208
209   If *secs* is not provided or :const:`None`, the current time as
210   returned by :func:`.time` is used. ``ctime(secs)`` is equivalent to
211   ``asctime(localtime(secs))``. Locale information is not used by
212   :func:`ctime`.
213
214
215.. function:: get_clock_info(name)
216
217   Get information on the specified clock as a namespace object.
218   Supported clock names and the corresponding functions to read their value
219   are:
220
221   * ``'clock'``: :func:`time.clock`
222   * ``'monotonic'``: :func:`time.monotonic`
223   * ``'perf_counter'``: :func:`time.perf_counter`
224   * ``'process_time'``: :func:`time.process_time`
225   * ``'thread_time'``: :func:`time.thread_time`
226   * ``'time'``: :func:`time.time`
227
228   The result has the following attributes:
229
230   - *adjustable*: ``True`` if the clock can be changed automatically (e.g. by
231     a NTP daemon) or manually by the system administrator, ``False`` otherwise
232   - *implementation*: The name of the underlying C function used to get
233     the clock value.  Refer to :ref:`time-clock-id-constants` for possible values.
234   - *monotonic*: ``True`` if the clock cannot go backward,
235     ``False`` otherwise
236   - *resolution*: The resolution of the clock in seconds (:class:`float`)
237
238   .. versionadded:: 3.3
239
240
241.. function:: gmtime([secs])
242
243   Convert a time expressed in seconds since the epoch to a :class:`struct_time` in
244   UTC in which the dst flag is always zero.  If *secs* is not provided or
245   :const:`None`, the current time as returned by :func:`.time` is used.  Fractions
246   of a second are ignored.  See above for a description of the
247   :class:`struct_time` object. See :func:`calendar.timegm` for the inverse of this
248   function.
249
250
251.. function:: localtime([secs])
252
253   Like :func:`gmtime` but converts to local time.  If *secs* is not provided or
254   :const:`None`, the current time as returned by :func:`.time` is used.  The dst
255   flag is set to ``1`` when DST applies to the given time.
256
257
258.. function:: mktime(t)
259
260   This is the inverse function of :func:`localtime`.  Its argument is the
261   :class:`struct_time` or full 9-tuple (since the dst flag is needed; use ``-1``
262   as the dst flag if it is unknown) which expresses the time in *local* time, not
263   UTC.  It returns a floating point number, for compatibility with :func:`.time`.
264   If the input value cannot be represented as a valid time, either
265   :exc:`OverflowError` or :exc:`ValueError` will be raised (which depends on
266   whether the invalid value is caught by Python or the underlying C libraries).
267   The earliest date for which it can generate a time is platform-dependent.
268
269
270.. function:: monotonic() -> float
271
272   Return the value (in fractional seconds) of a monotonic clock, i.e. a clock
273   that cannot go backwards.  The clock is not affected by system clock updates.
274   The reference point of the returned value is undefined, so that only the
275   difference between the results of consecutive calls is valid.
276
277   .. versionadded:: 3.3
278   .. versionchanged:: 3.5
279      The function is now always available and always system-wide.
280
281
282.. function:: monotonic_ns() -> int
283
284   Similar to :func:`monotonic`, but return time as nanoseconds.
285
286   .. versionadded:: 3.7
287
288.. function:: perf_counter() -> float
289
290   .. index::
291      single: benchmarking
292
293   Return the value (in fractional seconds) of a performance counter, i.e. a
294   clock with the highest available resolution to measure a short duration.  It
295   does include time elapsed during sleep and is system-wide.  The reference
296   point of the returned value is undefined, so that only the difference between
297   the results of consecutive calls is valid.
298
299   .. versionadded:: 3.3
300
301.. function:: perf_counter_ns() -> int
302
303   Similar to :func:`perf_counter`, but return time as nanoseconds.
304
305   .. versionadded:: 3.7
306
307
308.. function:: process_time() -> float
309
310   .. index::
311      single: CPU time
312      single: processor time
313      single: benchmarking
314
315   Return the value (in fractional seconds) of the sum of the system and user
316   CPU time of the current process.  It does not include time elapsed during
317   sleep.  It is process-wide by definition.  The reference point of the
318   returned value is undefined, so that only the difference between the results
319   of consecutive calls is valid.
320
321   .. versionadded:: 3.3
322
323.. function:: process_time_ns() -> int
324
325   Similar to :func:`process_time` but return time as nanoseconds.
326
327   .. versionadded:: 3.7
328
329.. function:: sleep(secs)
330
331   Suspend execution of the calling thread for the given number of seconds.
332   The argument may be a floating point number to indicate a more precise sleep
333   time. The actual suspension time may be less than that requested because any
334   caught signal will terminate the :func:`sleep` following execution of that
335   signal's catching routine.  Also, the suspension time may be longer than
336   requested by an arbitrary amount because of the scheduling of other activity
337   in the system.
338
339   .. versionchanged:: 3.5
340      The function now sleeps at least *secs* even if the sleep is interrupted
341      by a signal, except if the signal handler raises an exception (see
342      :pep:`475` for the rationale).
343
344
345.. index::
346   single: % (percent); datetime format
347
348.. function:: strftime(format[, t])
349
350   Convert a tuple or :class:`struct_time` representing a time as returned by
351   :func:`gmtime` or :func:`localtime` to a string as specified by the *format*
352   argument.  If *t* is not provided, the current time as returned by
353   :func:`localtime` is used.  *format* must be a string.  :exc:`ValueError` is
354   raised if any field in *t* is outside of the allowed range.
355
356   0 is a legal argument for any position in the time tuple; if it is normally
357   illegal the value is forced to a correct one.
358
359   The following directives can be embedded in the *format* string. They are shown
360   without the optional field width and precision specification, and are replaced
361   by the indicated characters in the :func:`strftime` result:
362
363   +-----------+------------------------------------------------+-------+
364   | Directive | Meaning                                        | Notes |
365   +===========+================================================+=======+
366   | ``%a``    | Locale's abbreviated weekday name.             |       |
367   |           |                                                |       |
368   +-----------+------------------------------------------------+-------+
369   | ``%A``    | Locale's full weekday name.                    |       |
370   +-----------+------------------------------------------------+-------+
371   | ``%b``    | Locale's abbreviated month name.               |       |
372   |           |                                                |       |
373   +-----------+------------------------------------------------+-------+
374   | ``%B``    | Locale's full month name.                      |       |
375   +-----------+------------------------------------------------+-------+
376   | ``%c``    | Locale's appropriate date and time             |       |
377   |           | representation.                                |       |
378   +-----------+------------------------------------------------+-------+
379   | ``%d``    | Day of the month as a decimal number [01,31].  |       |
380   |           |                                                |       |
381   +-----------+------------------------------------------------+-------+
382   | ``%H``    | Hour (24-hour clock) as a decimal number       |       |
383   |           | [00,23].                                       |       |
384   +-----------+------------------------------------------------+-------+
385   | ``%I``    | Hour (12-hour clock) as a decimal number       |       |
386   |           | [01,12].                                       |       |
387   +-----------+------------------------------------------------+-------+
388   | ``%j``    | Day of the year as a decimal number [001,366]. |       |
389   |           |                                                |       |
390   +-----------+------------------------------------------------+-------+
391   | ``%m``    | Month as a decimal number [01,12].             |       |
392   |           |                                                |       |
393   +-----------+------------------------------------------------+-------+
394   | ``%M``    | Minute as a decimal number [00,59].            |       |
395   |           |                                                |       |
396   +-----------+------------------------------------------------+-------+
397   | ``%p``    | Locale's equivalent of either AM or PM.        | \(1)  |
398   |           |                                                |       |
399   +-----------+------------------------------------------------+-------+
400   | ``%S``    | Second as a decimal number [00,61].            | \(2)  |
401   |           |                                                |       |
402   +-----------+------------------------------------------------+-------+
403   | ``%U``    | Week number of the year (Sunday as the first   | \(3)  |
404   |           | day of the week) as a decimal number [00,53].  |       |
405   |           | All days in a new year preceding the first     |       |
406   |           | Sunday are considered to be in week 0.         |       |
407   |           |                                                |       |
408   |           |                                                |       |
409   |           |                                                |       |
410   +-----------+------------------------------------------------+-------+
411   | ``%w``    | Weekday as a decimal number [0(Sunday),6].     |       |
412   |           |                                                |       |
413   +-----------+------------------------------------------------+-------+
414   | ``%W``    | Week number of the year (Monday as the first   | \(3)  |
415   |           | day of the week) as a decimal number [00,53].  |       |
416   |           | All days in a new year preceding the first     |       |
417   |           | Monday are considered to be in week 0.         |       |
418   |           |                                                |       |
419   |           |                                                |       |
420   |           |                                                |       |
421   +-----------+------------------------------------------------+-------+
422   | ``%x``    | Locale's appropriate date representation.      |       |
423   |           |                                                |       |
424   +-----------+------------------------------------------------+-------+
425   | ``%X``    | Locale's appropriate time representation.      |       |
426   |           |                                                |       |
427   +-----------+------------------------------------------------+-------+
428   | ``%y``    | Year without century as a decimal number       |       |
429   |           | [00,99].                                       |       |
430   +-----------+------------------------------------------------+-------+
431   | ``%Y``    | Year with century as a decimal number.         |       |
432   |           |                                                |       |
433   +-----------+------------------------------------------------+-------+
434   | ``%z``    | Time zone offset indicating a positive or      |       |
435   |           | negative time difference from UTC/GMT of the   |       |
436   |           | form +HHMM or -HHMM, where H represents decimal|       |
437   |           | hour digits and M represents decimal minute    |       |
438   |           | digits [-23:59, +23:59].                       |       |
439   +-----------+------------------------------------------------+-------+
440   | ``%Z``    | Time zone name (no characters if no time zone  |       |
441   |           | exists).                                       |       |
442   +-----------+------------------------------------------------+-------+
443   | ``%%``    | A literal ``'%'`` character.                   |       |
444   +-----------+------------------------------------------------+-------+
445
446   Notes:
447
448   (1)
449      When used with the :func:`strptime` function, the ``%p`` directive only affects
450      the output hour field if the ``%I`` directive is used to parse the hour.
451
452   (2)
453      The range really is ``0`` to ``61``; value ``60`` is valid in
454      timestamps representing `leap seconds`_ and value ``61`` is supported
455      for historical reasons.
456
457   (3)
458      When used with the :func:`strptime` function, ``%U`` and ``%W`` are only used in
459      calculations when the day of the week and the year are specified.
460
461   Here is an example, a format for dates compatible with that specified  in the
462   :rfc:`2822` Internet email standard.  [#]_ ::
463
464      >>> from time import gmtime, strftime
465      >>> strftime("%a, %d %b %Y %H:%M:%S +0000", gmtime())
466      'Thu, 28 Jun 2001 14:17:15 +0000'
467
468   Additional directives may be supported on certain platforms, but only the
469   ones listed here have a meaning standardized by ANSI C.  To see the full set
470   of format codes supported on your platform, consult the :manpage:`strftime(3)`
471   documentation.
472
473   On some platforms, an optional field width and precision specification can
474   immediately follow the initial ``'%'`` of a directive in the following order;
475   this is also not portable. The field width is normally 2 except for ``%j`` where
476   it is 3.
477
478
479.. index::
480   single: % (percent); datetime format
481
482.. function:: strptime(string[, format])
483
484   Parse a string representing a time according to a format.  The return value
485   is a :class:`struct_time` as returned by :func:`gmtime` or
486   :func:`localtime`.
487
488   The *format* parameter uses the same directives as those used by
489   :func:`strftime`; it defaults to ``"%a %b %d %H:%M:%S %Y"`` which matches the
490   formatting returned by :func:`ctime`. If *string* cannot be parsed according
491   to *format*, or if it has excess data after parsing, :exc:`ValueError` is
492   raised. The default values used to fill in any missing data when more
493   accurate values cannot be inferred are ``(1900, 1, 1, 0, 0, 0, 0, 1, -1)``.
494   Both *string* and *format* must be strings.
495
496   For example:
497
498      >>> import time
499      >>> time.strptime("30 Nov 00", "%d %b %y")   # doctest: +NORMALIZE_WHITESPACE
500      time.struct_time(tm_year=2000, tm_mon=11, tm_mday=30, tm_hour=0, tm_min=0,
501                       tm_sec=0, tm_wday=3, tm_yday=335, tm_isdst=-1)
502
503   Support for the ``%Z`` directive is based on the values contained in ``tzname``
504   and whether ``daylight`` is true.  Because of this, it is platform-specific
505   except for recognizing UTC and GMT which are always known (and are considered to
506   be non-daylight savings timezones).
507
508   Only the directives specified in the documentation are supported.  Because
509   ``strftime()`` is implemented per platform it can sometimes offer more
510   directives than those listed.  But ``strptime()`` is independent of any platform
511   and thus does not necessarily support all directives available that are not
512   documented as supported.
513
514
515.. class:: struct_time
516
517   The type of the time value sequence returned by :func:`gmtime`,
518   :func:`localtime`, and :func:`strptime`.  It is an object with a :term:`named
519   tuple` interface: values can be accessed by index and by attribute name.  The
520   following values are present:
521
522   +-------+-------------------+---------------------------------+
523   | Index | Attribute         | Values                          |
524   +=======+===================+=================================+
525   | 0     | :attr:`tm_year`   | (for example, 1993)             |
526   +-------+-------------------+---------------------------------+
527   | 1     | :attr:`tm_mon`    | range [1, 12]                   |
528   +-------+-------------------+---------------------------------+
529   | 2     | :attr:`tm_mday`   | range [1, 31]                   |
530   +-------+-------------------+---------------------------------+
531   | 3     | :attr:`tm_hour`   | range [0, 23]                   |
532   +-------+-------------------+---------------------------------+
533   | 4     | :attr:`tm_min`    | range [0, 59]                   |
534   +-------+-------------------+---------------------------------+
535   | 5     | :attr:`tm_sec`    | range [0, 61]; see **(2)** in   |
536   |       |                   | :func:`strftime` description    |
537   +-------+-------------------+---------------------------------+
538   | 6     | :attr:`tm_wday`   | range [0, 6], Monday is 0       |
539   +-------+-------------------+---------------------------------+
540   | 7     | :attr:`tm_yday`   | range [1, 366]                  |
541   +-------+-------------------+---------------------------------+
542   | 8     | :attr:`tm_isdst`  | 0, 1 or -1; see below           |
543   +-------+-------------------+---------------------------------+
544   | N/A   | :attr:`tm_zone`   | abbreviation of timezone name   |
545   +-------+-------------------+---------------------------------+
546   | N/A   | :attr:`tm_gmtoff` | offset east of UTC in seconds   |
547   +-------+-------------------+---------------------------------+
548
549   Note that unlike the C structure, the month value is a range of [1, 12], not
550   [0, 11].
551
552   In calls to :func:`mktime`, :attr:`tm_isdst` may be set to 1 when daylight
553   savings time is in effect, and 0 when it is not.  A value of -1 indicates that
554   this is not known, and will usually result in the correct state being filled in.
555
556   When a tuple with an incorrect length is passed to a function expecting a
557   :class:`struct_time`, or having elements of the wrong type, a
558   :exc:`TypeError` is raised.
559
560.. function:: time() -> float
561
562   Return the time in seconds since the epoch_ as a floating point
563   number. The specific date of the epoch and the handling of
564   `leap seconds`_ is platform dependent.
565   On Windows and most Unix systems, the epoch is January 1, 1970,
566   00:00:00 (UTC) and leap seconds are not counted towards the time
567   in seconds since the epoch. This is commonly referred to as
568   `Unix time <https://en.wikipedia.org/wiki/Unix_time>`_.
569   To find out what the epoch is on a given platform, look at
570   ``gmtime(0)``.
571
572   Note that even though the time is always returned as a floating point
573   number, not all systems provide time with a better precision than 1 second.
574   While this function normally returns non-decreasing values, it can return a
575   lower value than a previous call if the system clock has been set back
576   between the two calls.
577
578   The number returned by :func:`.time` may be converted into a more common
579   time format (i.e. year, month, day, hour, etc...) in UTC by passing it to
580   :func:`gmtime` function or in local time by passing it to the
581   :func:`localtime` function. In both cases a
582   :class:`struct_time` object is returned, from which the components
583   of the calendar date may be accessed as attributes.
584
585
586.. function:: thread_time() -> float
587
588   .. index::
589      single: CPU time
590      single: processor time
591      single: benchmarking
592
593   Return the value (in fractional seconds) of the sum of the system and user
594   CPU time of the current thread.  It does not include time elapsed during
595   sleep.  It is thread-specific by definition.  The reference point of the
596   returned value is undefined, so that only the difference between the results
597   of consecutive calls in the same thread is valid.
598
599   .. availability::  Windows, Linux, Unix systems supporting
600      ``CLOCK_THREAD_CPUTIME_ID``.
601
602   .. versionadded:: 3.7
603
604
605.. function:: thread_time_ns() -> int
606
607   Similar to :func:`thread_time` but return time as nanoseconds.
608
609   .. versionadded:: 3.7
610
611
612.. function:: time_ns() -> int
613
614   Similar to :func:`~time.time` but returns time as an integer number of nanoseconds
615   since the epoch_.
616
617   .. versionadded:: 3.7
618
619.. function:: tzset()
620
621   Reset the time conversion rules used by the library routines. The environment
622   variable :envvar:`TZ` specifies how this is done. It will also set the variables
623   ``tzname`` (from the :envvar:`TZ` environment variable), ``timezone`` (non-DST
624   seconds West of UTC), ``altzone`` (DST seconds west of UTC) and ``daylight``
625   (to 0 if this timezone does not have any daylight saving time rules, or to
626   nonzero if there is a time, past, present or future when daylight saving time
627   applies).
628
629   .. availability:: Unix.
630
631   .. note::
632
633      Although in many cases, changing the :envvar:`TZ` environment variable may
634      affect the output of functions like :func:`localtime` without calling
635      :func:`tzset`, this behavior should not be relied on.
636
637      The :envvar:`TZ` environment variable should contain no whitespace.
638
639   The standard format of the :envvar:`TZ` environment variable is (whitespace
640   added for clarity)::
641
642      std offset [dst [offset [,start[/time], end[/time]]]]
643
644   Where the components are:
645
646   ``std`` and ``dst``
647      Three or more alphanumerics giving the timezone abbreviations. These will be
648      propagated into time.tzname
649
650   ``offset``
651      The offset has the form: ``± hh[:mm[:ss]]``. This indicates the value
652      added the local time to arrive at UTC.  If preceded by a '-', the timezone
653      is east of the Prime Meridian; otherwise, it is west. If no offset follows
654      dst, summer time is assumed to be one hour ahead of standard time.
655
656   ``start[/time], end[/time]``
657      Indicates when to change to and back from DST. The format of the
658      start and end dates are one of the following:
659
660      :samp:`J{n}`
661         The Julian day *n* (1 <= *n* <= 365). Leap days are not counted, so in
662         all years February 28 is day 59 and March 1 is day 60.
663
664      :samp:`{n}`
665         The zero-based Julian day (0 <= *n* <= 365). Leap days are counted, and
666         it is possible to refer to February 29.
667
668      :samp:`M{m}.{n}.{d}`
669         The *d*'th day (0 <= *d* <= 6) of week *n* of month *m* of the year (1
670         <= *n* <= 5, 1 <= *m* <= 12, where week 5 means "the last *d* day in
671         month *m*" which may occur in either the fourth or the fifth
672         week). Week 1 is the first week in which the *d*'th day occurs. Day
673         zero is a Sunday.
674
675      ``time`` has the same format as ``offset`` except that no leading sign
676      ('-' or '+') is allowed. The default, if time is not given, is 02:00:00.
677
678   ::
679
680      >>> os.environ['TZ'] = 'EST+05EDT,M4.1.0,M10.5.0'
681      >>> time.tzset()
682      >>> time.strftime('%X %x %Z')
683      '02:07:36 05/08/03 EDT'
684      >>> os.environ['TZ'] = 'AEST-10AEDT-11,M10.5.0,M3.5.0'
685      >>> time.tzset()
686      >>> time.strftime('%X %x %Z')
687      '16:08:12 05/08/03 AEST'
688
689   On many Unix systems (including \*BSD, Linux, Solaris, and Darwin), it is more
690   convenient to use the system's zoneinfo (:manpage:`tzfile(5)`)  database to
691   specify the timezone rules. To do this, set the  :envvar:`TZ` environment
692   variable to the path of the required timezone  datafile, relative to the root of
693   the systems 'zoneinfo' timezone database, usually located at
694   :file:`/usr/share/zoneinfo`. For example,  ``'US/Eastern'``,
695   ``'Australia/Melbourne'``, ``'Egypt'`` or  ``'Europe/Amsterdam'``. ::
696
697      >>> os.environ['TZ'] = 'US/Eastern'
698      >>> time.tzset()
699      >>> time.tzname
700      ('EST', 'EDT')
701      >>> os.environ['TZ'] = 'Egypt'
702      >>> time.tzset()
703      >>> time.tzname
704      ('EET', 'EEST')
705
706
707.. _time-clock-id-constants:
708
709Clock ID Constants
710------------------
711
712These constants are used as parameters for :func:`clock_getres` and
713:func:`clock_gettime`.
714
715.. data:: CLOCK_BOOTTIME
716
717   Identical to :data:`CLOCK_MONOTONIC`, except it also includes any time that
718   the system is suspended.
719
720   This allows applications to get a suspend-aware monotonic  clock  without
721   having to deal with the complications of :data:`CLOCK_REALTIME`, which may
722   have  discontinuities if the time is changed using ``settimeofday()`` or
723   similar.
724
725   .. availability:: Linux 2.6.39 or later.
726
727   .. versionadded:: 3.7
728
729
730.. data:: CLOCK_HIGHRES
731
732   The Solaris OS has a ``CLOCK_HIGHRES`` timer that attempts to use an optimal
733   hardware source, and may give close to nanosecond resolution.
734   ``CLOCK_HIGHRES`` is the nonadjustable, high-resolution clock.
735
736   .. availability:: Solaris.
737
738   .. versionadded:: 3.3
739
740
741.. data:: CLOCK_MONOTONIC
742
743   Clock that cannot be set and represents monotonic time since some unspecified
744   starting point.
745
746   .. availability:: Unix.
747
748   .. versionadded:: 3.3
749
750
751.. data:: CLOCK_MONOTONIC_RAW
752
753   Similar to :data:`CLOCK_MONOTONIC`, but provides access to a raw
754   hardware-based time that is not subject to NTP adjustments.
755
756   .. availability:: Linux 2.6.28 and newer, macOS 10.12 and newer.
757
758   .. versionadded:: 3.3
759
760
761.. data:: CLOCK_PROCESS_CPUTIME_ID
762
763   High-resolution per-process timer from the CPU.
764
765   .. availability:: Unix.
766
767   .. versionadded:: 3.3
768
769
770.. data:: CLOCK_PROF
771
772   High-resolution per-process timer from the CPU.
773
774   .. availability:: FreeBSD, NetBSD 7 or later, OpenBSD.
775
776   .. versionadded:: 3.7
777
778
779.. data:: CLOCK_THREAD_CPUTIME_ID
780
781   Thread-specific CPU-time clock.
782
783   .. availability::  Unix.
784
785   .. versionadded:: 3.3
786
787
788.. data:: CLOCK_UPTIME
789
790   Time whose absolute value is the time the system has been running and not
791   suspended, providing accurate uptime measurement, both absolute and
792   interval.
793
794   .. availability:: FreeBSD, OpenBSD 5.5 or later.
795
796   .. versionadded:: 3.7
797
798
799.. data:: CLOCK_UPTIME_RAW
800
801   Clock that increments monotonically, tracking the time since an arbitrary
802   point, unaffected by frequency or time adjustments and not incremented while
803   the system is asleep.
804
805   .. availability:: macOS 10.12 and newer.
806
807   .. versionadded:: 3.8
808
809
810The following constant is the only parameter that can be sent to
811:func:`clock_settime`.
812
813
814.. data:: CLOCK_REALTIME
815
816   System-wide real-time clock.  Setting this clock requires appropriate
817   privileges.
818
819   .. availability:: Unix.
820
821   .. versionadded:: 3.3
822
823
824.. _time-timezone-constants:
825
826Timezone Constants
827-------------------
828
829.. data:: altzone
830
831   The offset of the local DST timezone, in seconds west of UTC, if one is defined.
832   This is negative if the local DST timezone is east of UTC (as in Western Europe,
833   including the UK).  Only use this if ``daylight`` is nonzero.  See note below.
834
835.. data:: daylight
836
837   Nonzero if a DST timezone is defined.  See note below.
838
839.. data:: timezone
840
841   The offset of the local (non-DST) timezone, in seconds west of UTC (negative in
842   most of Western Europe, positive in the US, zero in the UK).  See note below.
843
844.. data:: tzname
845
846   A tuple of two strings: the first is the name of the local non-DST timezone, the
847   second is the name of the local DST timezone.  If no DST timezone is defined,
848   the second string should not be used.  See note below.
849
850.. note::
851
852   For the above Timezone constants (:data:`altzone`, :data:`daylight`, :data:`timezone`,
853   and :data:`tzname`), the value is determined by the timezone rules in effect
854   at module load time or the last time :func:`tzset` is called and may be incorrect
855   for times in the past.  It is recommended to use the :attr:`tm_gmtoff` and
856   :attr:`tm_zone` results from :func:`localtime` to obtain timezone information.
857
858
859.. seealso::
860
861   Module :mod:`datetime`
862      More object-oriented interface to dates and times.
863
864   Module :mod:`locale`
865      Internationalization services.  The locale setting affects the interpretation
866      of many format specifiers in :func:`strftime` and :func:`strptime`.
867
868   Module :mod:`calendar`
869      General calendar-related functions.   :func:`~calendar.timegm` is the
870      inverse of :func:`gmtime` from this module.
871
872.. rubric:: Footnotes
873
874.. [#] The use of ``%Z`` is now deprecated, but the ``%z`` escape that expands to the
875   preferred  hour/minute offset is not supported by all ANSI C libraries. Also, a
876   strict reading of the original 1982 :rfc:`822` standard calls for a two-digit
877   year (%y rather than %Y), but practice moved to 4-digit years long before the
878   year 2000.  After that, :rfc:`822` became obsolete and the 4-digit year has
879   been first recommended by :rfc:`1123` and then mandated by :rfc:`2822`.
880
881