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 Use :func:`clock_gettime_ns` to avoid the precision loss caused by the 170 :class:`float` type. 171 172 .. availability:: Unix. 173 174 .. versionadded:: 3.3 175 176 177.. function:: clock_gettime_ns(clk_id) -> int 178 179 Similar to :func:`clock_gettime` but return time as nanoseconds. 180 181 .. availability:: Unix. 182 183 .. versionadded:: 3.7 184 185 186.. function:: clock_settime(clk_id, time: float) 187 188 Set the time of the specified clock *clk_id*. Currently, 189 :data:`CLOCK_REALTIME` is the only accepted value for *clk_id*. 190 191 Use :func:`clock_settime_ns` to avoid the precision loss caused by the 192 :class:`float` type. 193 194 .. availability:: Unix. 195 196 .. versionadded:: 3.3 197 198 199.. function:: clock_settime_ns(clk_id, time: int) 200 201 Similar to :func:`clock_settime` but set time with nanoseconds. 202 203 .. availability:: Unix. 204 205 .. versionadded:: 3.7 206 207 208.. function:: ctime([secs]) 209 210 Convert a time expressed in seconds since the epoch to a string of a form: 211 ``'Sun Jun 20 23:21:05 1993'`` representing local time. The day field 212 is two characters long and is space padded if the day is a single digit, 213 e.g.: ``'Wed Jun 9 04:26:40 1993'``. 214 215 If *secs* is not provided or :const:`None`, the current time as 216 returned by :func:`.time` is used. ``ctime(secs)`` is equivalent to 217 ``asctime(localtime(secs))``. Locale information is not used by 218 :func:`ctime`. 219 220 221.. function:: get_clock_info(name) 222 223 Get information on the specified clock as a namespace object. 224 Supported clock names and the corresponding functions to read their value 225 are: 226 227 * ``'monotonic'``: :func:`time.monotonic` 228 * ``'perf_counter'``: :func:`time.perf_counter` 229 * ``'process_time'``: :func:`time.process_time` 230 * ``'thread_time'``: :func:`time.thread_time` 231 * ``'time'``: :func:`time.time` 232 233 The result has the following attributes: 234 235 - *adjustable*: ``True`` if the clock can be changed automatically (e.g. by 236 a NTP daemon) or manually by the system administrator, ``False`` otherwise 237 - *implementation*: The name of the underlying C function used to get 238 the clock value. Refer to :ref:`time-clock-id-constants` for possible values. 239 - *monotonic*: ``True`` if the clock cannot go backward, 240 ``False`` otherwise 241 - *resolution*: The resolution of the clock in seconds (:class:`float`) 242 243 .. versionadded:: 3.3 244 245 246.. function:: gmtime([secs]) 247 248 Convert a time expressed in seconds since the epoch to a :class:`struct_time` in 249 UTC in which the dst flag is always zero. If *secs* is not provided or 250 :const:`None`, the current time as returned by :func:`.time` is used. Fractions 251 of a second are ignored. See above for a description of the 252 :class:`struct_time` object. See :func:`calendar.timegm` for the inverse of this 253 function. 254 255 256.. function:: localtime([secs]) 257 258 Like :func:`gmtime` but converts to local time. If *secs* is not provided or 259 :const:`None`, the current time as returned by :func:`.time` is used. The dst 260 flag is set to ``1`` when DST applies to the given time. 261 262 263.. function:: mktime(t) 264 265 This is the inverse function of :func:`localtime`. Its argument is the 266 :class:`struct_time` or full 9-tuple (since the dst flag is needed; use ``-1`` 267 as the dst flag if it is unknown) which expresses the time in *local* time, not 268 UTC. It returns a floating point number, for compatibility with :func:`.time`. 269 If the input value cannot be represented as a valid time, either 270 :exc:`OverflowError` or :exc:`ValueError` will be raised (which depends on 271 whether the invalid value is caught by Python or the underlying C libraries). 272 The earliest date for which it can generate a time is platform-dependent. 273 274 275.. function:: monotonic() -> float 276 277 Return the value (in fractional seconds) of a monotonic clock, i.e. a clock 278 that cannot go backwards. The clock is not affected by system clock updates. 279 The reference point of the returned value is undefined, so that only the 280 difference between the results of two calls is valid. 281 282 Use :func:`monotonic_ns` to avoid the precision loss caused by the 283 :class:`float` type. 284 285 .. versionadded:: 3.3 286 287 .. versionchanged:: 3.5 288 The function is now always available and always system-wide. 289 290 .. versionchanged:: 3.10 291 On macOS, the function is now system-wide. 292 293 294.. function:: monotonic_ns() -> int 295 296 Similar to :func:`monotonic`, but return time as nanoseconds. 297 298 .. versionadded:: 3.7 299 300.. function:: perf_counter() -> float 301 302 .. index:: 303 single: benchmarking 304 305 Return the value (in fractional seconds) of a performance counter, i.e. a 306 clock with the highest available resolution to measure a short duration. It 307 does include time elapsed during sleep and is system-wide. The reference 308 point of the returned value is undefined, so that only the difference between 309 the results of two calls is valid. 310 311 Use :func:`perf_counter_ns` to avoid the precision loss caused by the 312 :class:`float` type. 313 314 .. versionadded:: 3.3 315 316 .. versionchanged:: 3.10 317 On Windows, the function is now system-wide. 318 319.. function:: perf_counter_ns() -> int 320 321 Similar to :func:`perf_counter`, but return time as nanoseconds. 322 323 .. versionadded:: 3.7 324 325 326.. function:: process_time() -> float 327 328 .. index:: 329 single: CPU time 330 single: processor time 331 single: benchmarking 332 333 Return the value (in fractional seconds) of the sum of the system and user 334 CPU time of the current process. It does not include time elapsed during 335 sleep. It is process-wide by definition. The reference point of the 336 returned value is undefined, so that only the difference between the results 337 of two calls is valid. 338 339 Use :func:`process_time_ns` to avoid the precision loss caused by the 340 :class:`float` type. 341 342 .. versionadded:: 3.3 343 344.. function:: process_time_ns() -> int 345 346 Similar to :func:`process_time` but return time as nanoseconds. 347 348 .. versionadded:: 3.7 349 350.. function:: sleep(secs) 351 352 Suspend execution of the calling thread for the given number of seconds. 353 The argument may be a floating point number to indicate a more precise sleep 354 time. The actual suspension time may be less than that requested because any 355 caught signal will terminate the :func:`sleep` following execution of that 356 signal's catching routine. Also, the suspension time may be longer than 357 requested by an arbitrary amount because of the scheduling of other activity 358 in the system. 359 360 .. versionchanged:: 3.5 361 The function now sleeps at least *secs* even if the sleep is interrupted 362 by a signal, except if the signal handler raises an exception (see 363 :pep:`475` for the rationale). 364 365 366.. index:: 367 single: % (percent); datetime format 368 369.. function:: strftime(format[, t]) 370 371 Convert a tuple or :class:`struct_time` representing a time as returned by 372 :func:`gmtime` or :func:`localtime` to a string as specified by the *format* 373 argument. If *t* is not provided, the current time as returned by 374 :func:`localtime` is used. *format* must be a string. :exc:`ValueError` is 375 raised if any field in *t* is outside of the allowed range. 376 377 0 is a legal argument for any position in the time tuple; if it is normally 378 illegal the value is forced to a correct one. 379 380 The following directives can be embedded in the *format* string. They are shown 381 without the optional field width and precision specification, and are replaced 382 by the indicated characters in the :func:`strftime` result: 383 384 +-----------+------------------------------------------------+-------+ 385 | Directive | Meaning | Notes | 386 +===========+================================================+=======+ 387 | ``%a`` | Locale's abbreviated weekday name. | | 388 | | | | 389 +-----------+------------------------------------------------+-------+ 390 | ``%A`` | Locale's full weekday name. | | 391 +-----------+------------------------------------------------+-------+ 392 | ``%b`` | Locale's abbreviated month name. | | 393 | | | | 394 +-----------+------------------------------------------------+-------+ 395 | ``%B`` | Locale's full month name. | | 396 +-----------+------------------------------------------------+-------+ 397 | ``%c`` | Locale's appropriate date and time | | 398 | | representation. | | 399 +-----------+------------------------------------------------+-------+ 400 | ``%d`` | Day of the month as a decimal number [01,31]. | | 401 | | | | 402 +-----------+------------------------------------------------+-------+ 403 | ``%H`` | Hour (24-hour clock) as a decimal number | | 404 | | [00,23]. | | 405 +-----------+------------------------------------------------+-------+ 406 | ``%I`` | Hour (12-hour clock) as a decimal number | | 407 | | [01,12]. | | 408 +-----------+------------------------------------------------+-------+ 409 | ``%j`` | Day of the year as a decimal number [001,366]. | | 410 | | | | 411 +-----------+------------------------------------------------+-------+ 412 | ``%m`` | Month as a decimal number [01,12]. | | 413 | | | | 414 +-----------+------------------------------------------------+-------+ 415 | ``%M`` | Minute as a decimal number [00,59]. | | 416 | | | | 417 +-----------+------------------------------------------------+-------+ 418 | ``%p`` | Locale's equivalent of either AM or PM. | \(1) | 419 | | | | 420 +-----------+------------------------------------------------+-------+ 421 | ``%S`` | Second as a decimal number [00,61]. | \(2) | 422 | | | | 423 +-----------+------------------------------------------------+-------+ 424 | ``%U`` | Week number of the year (Sunday as the first | \(3) | 425 | | day of the week) as a decimal number [00,53]. | | 426 | | All days in a new year preceding the first | | 427 | | Sunday are considered to be in week 0. | | 428 | | | | 429 | | | | 430 | | | | 431 +-----------+------------------------------------------------+-------+ 432 | ``%w`` | Weekday as a decimal number [0(Sunday),6]. | | 433 | | | | 434 +-----------+------------------------------------------------+-------+ 435 | ``%W`` | Week number of the year (Monday as the first | \(3) | 436 | | day of the week) as a decimal number [00,53]. | | 437 | | All days in a new year preceding the first | | 438 | | Monday are considered to be in week 0. | | 439 | | | | 440 | | | | 441 | | | | 442 +-----------+------------------------------------------------+-------+ 443 | ``%x`` | Locale's appropriate date representation. | | 444 | | | | 445 +-----------+------------------------------------------------+-------+ 446 | ``%X`` | Locale's appropriate time representation. | | 447 | | | | 448 +-----------+------------------------------------------------+-------+ 449 | ``%y`` | Year without century as a decimal number | | 450 | | [00,99]. | | 451 +-----------+------------------------------------------------+-------+ 452 | ``%Y`` | Year with century as a decimal number. | | 453 | | | | 454 +-----------+------------------------------------------------+-------+ 455 | ``%z`` | Time zone offset indicating a positive or | | 456 | | negative time difference from UTC/GMT of the | | 457 | | form +HHMM or -HHMM, where H represents decimal| | 458 | | hour digits and M represents decimal minute | | 459 | | digits [-23:59, +23:59]. | | 460 +-----------+------------------------------------------------+-------+ 461 | ``%Z`` | Time zone name (no characters if no time zone | | 462 | | exists). | | 463 +-----------+------------------------------------------------+-------+ 464 | ``%%`` | A literal ``'%'`` character. | | 465 +-----------+------------------------------------------------+-------+ 466 467 Notes: 468 469 (1) 470 When used with the :func:`strptime` function, the ``%p`` directive only affects 471 the output hour field if the ``%I`` directive is used to parse the hour. 472 473 (2) 474 The range really is ``0`` to ``61``; value ``60`` is valid in 475 timestamps representing `leap seconds`_ and value ``61`` is supported 476 for historical reasons. 477 478 (3) 479 When used with the :func:`strptime` function, ``%U`` and ``%W`` are only used in 480 calculations when the day of the week and the year are specified. 481 482 Here is an example, a format for dates compatible with that specified in the 483 :rfc:`2822` Internet email standard. [#]_ :: 484 485 >>> from time import gmtime, strftime 486 >>> strftime("%a, %d %b %Y %H:%M:%S +0000", gmtime()) 487 'Thu, 28 Jun 2001 14:17:15 +0000' 488 489 Additional directives may be supported on certain platforms, but only the 490 ones listed here have a meaning standardized by ANSI C. To see the full set 491 of format codes supported on your platform, consult the :manpage:`strftime(3)` 492 documentation. 493 494 On some platforms, an optional field width and precision specification can 495 immediately follow the initial ``'%'`` of a directive in the following order; 496 this is also not portable. The field width is normally 2 except for ``%j`` where 497 it is 3. 498 499 500.. index:: 501 single: % (percent); datetime format 502 503.. function:: strptime(string[, format]) 504 505 Parse a string representing a time according to a format. The return value 506 is a :class:`struct_time` as returned by :func:`gmtime` or 507 :func:`localtime`. 508 509 The *format* parameter uses the same directives as those used by 510 :func:`strftime`; it defaults to ``"%a %b %d %H:%M:%S %Y"`` which matches the 511 formatting returned by :func:`ctime`. If *string* cannot be parsed according 512 to *format*, or if it has excess data after parsing, :exc:`ValueError` is 513 raised. The default values used to fill in any missing data when more 514 accurate values cannot be inferred are ``(1900, 1, 1, 0, 0, 0, 0, 1, -1)``. 515 Both *string* and *format* must be strings. 516 517 For example: 518 519 >>> import time 520 >>> time.strptime("30 Nov 00", "%d %b %y") # doctest: +NORMALIZE_WHITESPACE 521 time.struct_time(tm_year=2000, tm_mon=11, tm_mday=30, tm_hour=0, tm_min=0, 522 tm_sec=0, tm_wday=3, tm_yday=335, tm_isdst=-1) 523 524 Support for the ``%Z`` directive is based on the values contained in ``tzname`` 525 and whether ``daylight`` is true. Because of this, it is platform-specific 526 except for recognizing UTC and GMT which are always known (and are considered to 527 be non-daylight savings timezones). 528 529 Only the directives specified in the documentation are supported. Because 530 ``strftime()`` is implemented per platform it can sometimes offer more 531 directives than those listed. But ``strptime()`` is independent of any platform 532 and thus does not necessarily support all directives available that are not 533 documented as supported. 534 535 536.. class:: struct_time 537 538 The type of the time value sequence returned by :func:`gmtime`, 539 :func:`localtime`, and :func:`strptime`. It is an object with a :term:`named 540 tuple` interface: values can be accessed by index and by attribute name. The 541 following values are present: 542 543 +-------+-------------------+---------------------------------+ 544 | Index | Attribute | Values | 545 +=======+===================+=================================+ 546 | 0 | :attr:`tm_year` | (for example, 1993) | 547 +-------+-------------------+---------------------------------+ 548 | 1 | :attr:`tm_mon` | range [1, 12] | 549 +-------+-------------------+---------------------------------+ 550 | 2 | :attr:`tm_mday` | range [1, 31] | 551 +-------+-------------------+---------------------------------+ 552 | 3 | :attr:`tm_hour` | range [0, 23] | 553 +-------+-------------------+---------------------------------+ 554 | 4 | :attr:`tm_min` | range [0, 59] | 555 +-------+-------------------+---------------------------------+ 556 | 5 | :attr:`tm_sec` | range [0, 61]; see **(2)** in | 557 | | | :func:`strftime` description | 558 +-------+-------------------+---------------------------------+ 559 | 6 | :attr:`tm_wday` | range [0, 6], Monday is 0 | 560 +-------+-------------------+---------------------------------+ 561 | 7 | :attr:`tm_yday` | range [1, 366] | 562 +-------+-------------------+---------------------------------+ 563 | 8 | :attr:`tm_isdst` | 0, 1 or -1; see below | 564 +-------+-------------------+---------------------------------+ 565 | N/A | :attr:`tm_zone` | abbreviation of timezone name | 566 +-------+-------------------+---------------------------------+ 567 | N/A | :attr:`tm_gmtoff` | offset east of UTC in seconds | 568 +-------+-------------------+---------------------------------+ 569 570 Note that unlike the C structure, the month value is a range of [1, 12], not 571 [0, 11]. 572 573 In calls to :func:`mktime`, :attr:`tm_isdst` may be set to 1 when daylight 574 savings time is in effect, and 0 when it is not. A value of -1 indicates that 575 this is not known, and will usually result in the correct state being filled in. 576 577 When a tuple with an incorrect length is passed to a function expecting a 578 :class:`struct_time`, or having elements of the wrong type, a 579 :exc:`TypeError` is raised. 580 581.. function:: time() -> float 582 583 Return the time in seconds since the epoch_ as a floating point 584 number. The specific date of the epoch and the handling of 585 `leap seconds`_ is platform dependent. 586 On Windows and most Unix systems, the epoch is January 1, 1970, 587 00:00:00 (UTC) and leap seconds are not counted towards the time 588 in seconds since the epoch. This is commonly referred to as 589 `Unix time <https://en.wikipedia.org/wiki/Unix_time>`_. 590 To find out what the epoch is on a given platform, look at 591 ``gmtime(0)``. 592 593 Note that even though the time is always returned as a floating point 594 number, not all systems provide time with a better precision than 1 second. 595 While this function normally returns non-decreasing values, it can return a 596 lower value than a previous call if the system clock has been set back 597 between the two calls. 598 599 The number returned by :func:`.time` may be converted into a more common 600 time format (i.e. year, month, day, hour, etc...) in UTC by passing it to 601 :func:`gmtime` function or in local time by passing it to the 602 :func:`localtime` function. In both cases a 603 :class:`struct_time` object is returned, from which the components 604 of the calendar date may be accessed as attributes. 605 606 Use :func:`time_ns` to avoid the precision loss caused by the :class:`float` 607 type. 608 609 610.. function:: time_ns() -> int 611 612 Similar to :func:`~time.time` but returns time as an integer number of nanoseconds 613 since the epoch_. 614 615 .. versionadded:: 3.7 616 617 618.. function:: thread_time() -> float 619 620 .. index:: 621 single: CPU time 622 single: processor time 623 single: benchmarking 624 625 Return the value (in fractional seconds) of the sum of the system and user 626 CPU time of the current thread. It does not include time elapsed during 627 sleep. It is thread-specific by definition. The reference point of the 628 returned value is undefined, so that only the difference between the results 629 of two calls in the same thread is valid. 630 631 Use :func:`thread_time_ns` to avoid the precision loss caused by the 632 :class:`float` type. 633 634 .. availability:: Windows, Linux, Unix systems supporting 635 ``CLOCK_THREAD_CPUTIME_ID``. 636 637 .. versionadded:: 3.7 638 639 640.. function:: thread_time_ns() -> int 641 642 Similar to :func:`thread_time` but return time as nanoseconds. 643 644 .. versionadded:: 3.7 645 646 647.. function:: tzset() 648 649 Reset the time conversion rules used by the library routines. The environment 650 variable :envvar:`TZ` specifies how this is done. It will also set the variables 651 ``tzname`` (from the :envvar:`TZ` environment variable), ``timezone`` (non-DST 652 seconds West of UTC), ``altzone`` (DST seconds west of UTC) and ``daylight`` 653 (to 0 if this timezone does not have any daylight saving time rules, or to 654 nonzero if there is a time, past, present or future when daylight saving time 655 applies). 656 657 .. availability:: Unix. 658 659 .. note:: 660 661 Although in many cases, changing the :envvar:`TZ` environment variable may 662 affect the output of functions like :func:`localtime` without calling 663 :func:`tzset`, this behavior should not be relied on. 664 665 The :envvar:`TZ` environment variable should contain no whitespace. 666 667 The standard format of the :envvar:`TZ` environment variable is (whitespace 668 added for clarity):: 669 670 std offset [dst [offset [,start[/time], end[/time]]]] 671 672 Where the components are: 673 674 ``std`` and ``dst`` 675 Three or more alphanumerics giving the timezone abbreviations. These will be 676 propagated into time.tzname 677 678 ``offset`` 679 The offset has the form: ``± hh[:mm[:ss]]``. This indicates the value 680 added the local time to arrive at UTC. If preceded by a '-', the timezone 681 is east of the Prime Meridian; otherwise, it is west. If no offset follows 682 dst, summer time is assumed to be one hour ahead of standard time. 683 684 ``start[/time], end[/time]`` 685 Indicates when to change to and back from DST. The format of the 686 start and end dates are one of the following: 687 688 :samp:`J{n}` 689 The Julian day *n* (1 <= *n* <= 365). Leap days are not counted, so in 690 all years February 28 is day 59 and March 1 is day 60. 691 692 :samp:`{n}` 693 The zero-based Julian day (0 <= *n* <= 365). Leap days are counted, and 694 it is possible to refer to February 29. 695 696 :samp:`M{m}.{n}.{d}` 697 The *d*'th day (0 <= *d* <= 6) of week *n* of month *m* of the year (1 698 <= *n* <= 5, 1 <= *m* <= 12, where week 5 means "the last *d* day in 699 month *m*" which may occur in either the fourth or the fifth 700 week). Week 1 is the first week in which the *d*'th day occurs. Day 701 zero is a Sunday. 702 703 ``time`` has the same format as ``offset`` except that no leading sign 704 ('-' or '+') is allowed. The default, if time is not given, is 02:00:00. 705 706 :: 707 708 >>> os.environ['TZ'] = 'EST+05EDT,M4.1.0,M10.5.0' 709 >>> time.tzset() 710 >>> time.strftime('%X %x %Z') 711 '02:07:36 05/08/03 EDT' 712 >>> os.environ['TZ'] = 'AEST-10AEDT-11,M10.5.0,M3.5.0' 713 >>> time.tzset() 714 >>> time.strftime('%X %x %Z') 715 '16:08:12 05/08/03 AEST' 716 717 On many Unix systems (including \*BSD, Linux, Solaris, and Darwin), it is more 718 convenient to use the system's zoneinfo (:manpage:`tzfile(5)`) database to 719 specify the timezone rules. To do this, set the :envvar:`TZ` environment 720 variable to the path of the required timezone datafile, relative to the root of 721 the systems 'zoneinfo' timezone database, usually located at 722 :file:`/usr/share/zoneinfo`. For example, ``'US/Eastern'``, 723 ``'Australia/Melbourne'``, ``'Egypt'`` or ``'Europe/Amsterdam'``. :: 724 725 >>> os.environ['TZ'] = 'US/Eastern' 726 >>> time.tzset() 727 >>> time.tzname 728 ('EST', 'EDT') 729 >>> os.environ['TZ'] = 'Egypt' 730 >>> time.tzset() 731 >>> time.tzname 732 ('EET', 'EEST') 733 734 735.. _time-clock-id-constants: 736 737Clock ID Constants 738------------------ 739 740These constants are used as parameters for :func:`clock_getres` and 741:func:`clock_gettime`. 742 743.. data:: CLOCK_BOOTTIME 744 745 Identical to :data:`CLOCK_MONOTONIC`, except it also includes any time that 746 the system is suspended. 747 748 This allows applications to get a suspend-aware monotonic clock without 749 having to deal with the complications of :data:`CLOCK_REALTIME`, which may 750 have discontinuities if the time is changed using ``settimeofday()`` or 751 similar. 752 753 .. availability:: Linux 2.6.39 or later. 754 755 .. versionadded:: 3.7 756 757 758.. data:: CLOCK_HIGHRES 759 760 The Solaris OS has a ``CLOCK_HIGHRES`` timer that attempts to use an optimal 761 hardware source, and may give close to nanosecond resolution. 762 ``CLOCK_HIGHRES`` is the nonadjustable, high-resolution clock. 763 764 .. availability:: Solaris. 765 766 .. versionadded:: 3.3 767 768 769.. data:: CLOCK_MONOTONIC 770 771 Clock that cannot be set and represents monotonic time since some unspecified 772 starting point. 773 774 .. availability:: Unix. 775 776 .. versionadded:: 3.3 777 778 779.. data:: CLOCK_MONOTONIC_RAW 780 781 Similar to :data:`CLOCK_MONOTONIC`, but provides access to a raw 782 hardware-based time that is not subject to NTP adjustments. 783 784 .. availability:: Linux 2.6.28 and newer, macOS 10.12 and newer. 785 786 .. versionadded:: 3.3 787 788 789.. data:: CLOCK_PROCESS_CPUTIME_ID 790 791 High-resolution per-process timer from the CPU. 792 793 .. availability:: Unix. 794 795 .. versionadded:: 3.3 796 797 798.. data:: CLOCK_PROF 799 800 High-resolution per-process timer from the CPU. 801 802 .. availability:: FreeBSD, NetBSD 7 or later, OpenBSD. 803 804 .. versionadded:: 3.7 805 806.. data:: CLOCK_TAI 807 808 `International Atomic Time <https://www.nist.gov/pml/time-and-frequency-division/nist-time-frequently-asked-questions-faq#tai>`_ 809 810 The system must have a current leap second table in order for this to give 811 the correct answer. PTP or NTP software can maintain a leap second table. 812 813 .. availability:: Linux. 814 815 .. versionadded:: 3.9 816 817.. data:: CLOCK_THREAD_CPUTIME_ID 818 819 Thread-specific CPU-time clock. 820 821 .. availability:: Unix. 822 823 .. versionadded:: 3.3 824 825 826.. data:: CLOCK_UPTIME 827 828 Time whose absolute value is the time the system has been running and not 829 suspended, providing accurate uptime measurement, both absolute and 830 interval. 831 832 .. availability:: FreeBSD, OpenBSD 5.5 or later. 833 834 .. versionadded:: 3.7 835 836 837.. data:: CLOCK_UPTIME_RAW 838 839 Clock that increments monotonically, tracking the time since an arbitrary 840 point, unaffected by frequency or time adjustments and not incremented while 841 the system is asleep. 842 843 .. availability:: macOS 10.12 and newer. 844 845 .. versionadded:: 3.8 846 847The following constant is the only parameter that can be sent to 848:func:`clock_settime`. 849 850 851.. data:: CLOCK_REALTIME 852 853 System-wide real-time clock. Setting this clock requires appropriate 854 privileges. 855 856 .. availability:: Unix. 857 858 .. versionadded:: 3.3 859 860 861.. _time-timezone-constants: 862 863Timezone Constants 864------------------- 865 866.. data:: altzone 867 868 The offset of the local DST timezone, in seconds west of UTC, if one is defined. 869 This is negative if the local DST timezone is east of UTC (as in Western Europe, 870 including the UK). Only use this if ``daylight`` is nonzero. See note below. 871 872.. data:: daylight 873 874 Nonzero if a DST timezone is defined. See note below. 875 876.. data:: timezone 877 878 The offset of the local (non-DST) timezone, in seconds west of UTC (negative in 879 most of Western Europe, positive in the US, zero in the UK). See note below. 880 881.. data:: tzname 882 883 A tuple of two strings: the first is the name of the local non-DST timezone, the 884 second is the name of the local DST timezone. If no DST timezone is defined, 885 the second string should not be used. See note below. 886 887.. note:: 888 889 For the above Timezone constants (:data:`altzone`, :data:`daylight`, :data:`timezone`, 890 and :data:`tzname`), the value is determined by the timezone rules in effect 891 at module load time or the last time :func:`tzset` is called and may be incorrect 892 for times in the past. It is recommended to use the :attr:`tm_gmtoff` and 893 :attr:`tm_zone` results from :func:`localtime` to obtain timezone information. 894 895 896.. seealso:: 897 898 Module :mod:`datetime` 899 More object-oriented interface to dates and times. 900 901 Module :mod:`locale` 902 Internationalization services. The locale setting affects the interpretation 903 of many format specifiers in :func:`strftime` and :func:`strptime`. 904 905 Module :mod:`calendar` 906 General calendar-related functions. :func:`~calendar.timegm` is the 907 inverse of :func:`gmtime` from this module. 908 909.. rubric:: Footnotes 910 911.. [#] The use of ``%Z`` is now deprecated, but the ``%z`` escape that expands to the 912 preferred hour/minute offset is not supported by all ANSI C libraries. Also, a 913 strict reading of the original 1982 :rfc:`822` standard calls for a two-digit 914 year (%y rather than %Y), but practice moved to 4-digit years long before the 915 year 2000. After that, :rfc:`822` became obsolete and the 4-digit year has 916 been first recommended by :rfc:`1123` and then mandated by :rfc:`2822`. 917 918