16.3. "time" — Time access and conversions
******************************************

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

This module provides various time-related functions. For related
functionality, see also the "datetime" and "calendar" modules.

Although this module is always available, not all functions are
available on all platforms.  Most of the functions defined in this
module call platform C library functions with the same name.  It may
sometimes be helpful to consult the platform documentation, because
the semantics of these functions varies among platforms.

An explanation of some terminology and conventions is in order.

* The *epoch* is the point where the time starts, and is platform
  dependent.  For Unix, the epoch is January 1, 1970, 00:00:00 (UTC).
  To find out what the epoch is on a given platform, look at
  "time.gmtime(0)".

* The term *seconds since the epoch* refers to the total number of
  elapsed seconds since the epoch, typically excluding leap seconds.
  Leap seconds are excluded from this total on all POSIX-compliant
  platforms.

* The functions in this module may not handle dates and times before
  the epoch or far in the future.  The cut-off point in the future is
  determined by the C library; for 32-bit systems, it is typically in
  2038.

* **Year 2000 (Y2K) issues**: Python depends on the platform’s C
  library, which generally doesn’t have year 2000 issues, since all
  dates and times are represented internally as seconds since the
  epoch.  Function "strptime()" can parse 2-digit years when given
  "%y" format code.  When 2-digit years are parsed, they are converted
  according to the POSIX and ISO C standards: values 69–99 are mapped
  to 1969–1999, and values 0–68 are mapped to 2000–2068.

* UTC is Coordinated Universal Time (formerly known as Greenwich
  Mean Time, or GMT).  The acronym UTC is not a mistake but a
  compromise between English and French.

* DST is Daylight Saving Time, an adjustment of the timezone by
  (usually) one hour during part of the year.  DST rules are magic
  (determined by local law) and can change from year to year.  The C
  library has a table containing the local rules (often it is read
  from a system file for flexibility) and is the only source of True
  Wisdom in this respect.

* The precision of the various real-time functions may be less than
  suggested by the units in which their value or argument is
  expressed. E.g. on most Unix systems, the clock “ticks” only 50 or
  100 times a second.

* On the other hand, the precision of "time()" and "sleep()" is
  better than their Unix equivalents: times are expressed as floating
  point numbers, "time()" returns the most accurate time available
  (using Unix "gettimeofday()" where available), and "sleep()" will
  accept a time with a nonzero fraction (Unix "select()" is used to
  implement this, where available).

* The time value as returned by "gmtime()", "localtime()", and
  "strptime()", and accepted by "asctime()", "mktime()" and
  "strftime()", is a sequence of 9 integers.  The return values of
  "gmtime()", "localtime()", and "strptime()" also offer attribute
  names for individual fields.

  See "struct_time" for a description of these objects.

  Changed in version 3.3: The "struct_time" type was extended to
  provide the "tm_gmtoff" and "tm_zone" attributes when platform
  supports corresponding "struct tm" members.

* Use the following functions to convert between time
  representations:

  +---------------------------+---------------------------+---------------------------+
  | From                      | To                        | Use                       |
  +===========================+===========================+===========================+
  | seconds since the epoch   | "struct_time" in UTC      | "gmtime()"                |
  +---------------------------+---------------------------+---------------------------+
  | seconds since the epoch   | "struct_time" in local    | "localtime()"             |
  |                           | time                      |                           |
  +---------------------------+---------------------------+---------------------------+
  | "struct_time" in UTC      | seconds since the epoch   | "calendar.timegm()"       |
  +---------------------------+---------------------------+---------------------------+
  | "struct_time" in local    | seconds since the epoch   | "mktime()"                |
  | time                      |                           |                           |
  +---------------------------+---------------------------+---------------------------+

The module defines the following functions and data items:

time.altzone

   The offset of the local DST timezone, in seconds west of UTC, if
   one is defined. This is negative if the local DST timezone is east
   of UTC (as in Western Europe, including the UK).  Only use this if
   "daylight" is nonzero.

time.asctime([t])

   Convert a tuple or "struct_time" representing a time as returned by
   "gmtime()" or "localtime()" to a string of the following form:
   "'Sun Jun 20 23:21:05 1993'".  If *t* is not provided, the current
   time as returned by "localtime()" is used. Locale information is
   not used by "asctime()".

   Note: Unlike the C function of the same name, "asctime()" does
     not add a trailing newline.

time.clock()

   On Unix, return the current processor time as a floating point
   number expressed in seconds.  The precision, and in fact the very
   definition of the meaning of “processor time”, depends on that of
   the C function of the same name.

   On Windows, this function returns wall-clock seconds elapsed since
   the first call to this function, as a floating point number, based
   on the Win32 function "QueryPerformanceCounter()". The resolution
   is typically better than one microsecond.

   Deprecated since version 3.3: The behaviour of this function
   depends on the platform: use "perf_counter()" or "process_time()"
   instead, depending on your requirements, to have a well defined
   behaviour.

time.clock_getres(clk_id)

   Return the resolution (precision) of the specified clock *clk_id*.

   Availability: Unix.

   New in version 3.3.

time.clock_gettime(clk_id)

   Return the time of the specified clock *clk_id*.

   Availability: Unix.

   New in version 3.3.

time.clock_settime(clk_id, time)

   Set the time of the specified clock *clk_id*.

   Availability: Unix.

   New in version 3.3.

time.CLOCK_HIGHRES

   The Solaris OS has a CLOCK_HIGHRES timer that attempts to use an
   optimal hardware source, and may give close to nanosecond
   resolution.  CLOCK_HIGHRES is the nonadjustable, high-resolution
   clock.

   Availability: Solaris.

   New in version 3.3.

time.CLOCK_MONOTONIC

   Clock that cannot be set and represents monotonic time since some
   unspecified starting point.

   Availability: Unix.

   New in version 3.3.

time.CLOCK_MONOTONIC_RAW

   Similar to "CLOCK_MONOTONIC", but provides access to a raw
   hardware-based time that is not subject to NTP adjustments.

   Availability: Linux 2.6.28 or later.

   New in version 3.3.

time.CLOCK_PROCESS_CPUTIME_ID

   High-resolution per-process timer from the CPU.

   Availability: Unix.

   New in version 3.3.

time.CLOCK_REALTIME

   System-wide real-time clock.  Setting this clock requires
   appropriate privileges.

   Availability: Unix.

   New in version 3.3.

time.CLOCK_THREAD_CPUTIME_ID

   Thread-specific CPU-time clock.

   Availability: Unix.

   New in version 3.3.

time.ctime([secs])

   Convert a time expressed in seconds since the epoch to a string
   representing local time. If *secs* is not provided or "None", the
   current time as returned by "time()" is used.  "ctime(secs)" is
   equivalent to "asctime(localtime(secs))". Locale information is not
   used by "ctime()".

time.daylight

   Nonzero if a DST timezone is defined.

time.get_clock_info(name)

   Get information on the specified clock as a namespace object.
   Supported clock names and the corresponding functions to read their
   value are:

   * "'clock'": "time.clock()"

   * "'monotonic'": "time.monotonic()"

   * "'perf_counter'": "time.perf_counter()"

   * "'process_time'": "time.process_time()"

   * "'time'": "time.time()"

   The result has the following attributes:

   * *adjustable*: "True" if the clock can be changed automatically
     (e.g. by a NTP daemon) or manually by the system administrator,
     "False" otherwise

   * *implementation*: The name of the underlying C function used to
     get the clock value

   * *monotonic*: "True" if the clock cannot go backward, "False"
     otherwise

   * *resolution*: The resolution of the clock in seconds ("float")

   New in version 3.3.

time.gmtime([secs])

   Convert a time expressed in seconds since the epoch to a
   "struct_time" in UTC in which the dst flag is always zero.  If
   *secs* is not provided or "None", the current time as returned by
   "time()" is used.  Fractions of a second are ignored.  See above
   for a description of the "struct_time" object. See
   "calendar.timegm()" for the inverse of this function.

time.localtime([secs])

   Like "gmtime()" but converts to local time.  If *secs* is not
   provided or "None", the current time as returned by "time()" is
   used.  The dst flag is set to "1" when DST applies to the given
   time.

time.mktime(t)

   This is the inverse function of "localtime()".  Its argument is the
   "struct_time" or full 9-tuple (since the dst flag is needed; use
   "-1" as the dst flag if it is unknown) which expresses the time in
   *local* time, not UTC.  It returns a floating point number, for
   compatibility with "time()". If the input value cannot be
   represented as a valid time, either "OverflowError" or "ValueError"
   will be raised (which depends on whether the invalid value is
   caught by Python or the underlying C libraries). The earliest date
   for which it can generate a time is platform-dependent.

time.monotonic()

   Return the value (in fractional seconds) of a monotonic clock, i.e.
   a clock that cannot go backwards.  The clock is not affected by
   system clock updates. The reference point of the returned value is
   undefined, so that only the difference between the results of
   consecutive calls is valid.

   On Windows versions older than Vista, "monotonic()" detects
   "GetTickCount()" integer overflow (32 bits, roll-over after 49.7
   days). It increases an internal epoch (reference time) by 2^32 each
   time that an overflow is detected.  The epoch is stored in the
   process-local state and so the value of "monotonic()" may be
   different in two Python processes running for more than 49 days. On
   more recent versions of Windows and on other operating systems,
   "monotonic()" is system-wide.

   New in version 3.3.

   Changed in version 3.5: The function is now always available.

time.perf_counter()

   Return the value (in fractional seconds) of a performance counter,
   i.e. a clock with the highest available resolution to measure a
   short duration.  It does include time elapsed during sleep and is
   system-wide.  The reference point of the returned value is
   undefined, so that only the difference between the results of
   consecutive calls is valid.

   New in version 3.3.

time.process_time()

   Return the value (in fractional seconds) of the sum of the system
   and user CPU time of the current process.  It does not include time
   elapsed during sleep.  It is process-wide by definition.  The
   reference point of the returned value is undefined, so that only
   the difference between the results of consecutive calls is valid.

   New in version 3.3.

time.sleep(secs)

   Suspend execution of the calling thread for the given number of
   seconds. The argument may be a floating point number to indicate a
   more precise sleep time. The actual suspension time may be less
   than that requested because any caught signal will terminate the
   "sleep()" following execution of that signal’s catching routine.
   Also, the suspension time may be longer than requested by an
   arbitrary amount because of the scheduling of other activity in the
   system.

   Changed in version 3.5: The function now sleeps at least *secs*
   even if the sleep is interrupted by a signal, except if the signal
   handler raises an exception (see **PEP 475** for the rationale).

time.strftime(format[, t])

   Convert a tuple or "struct_time" representing a time as returned by
   "gmtime()" or "localtime()" to a string as specified by the
   *format* argument.  If *t* is not provided, the current time as
   returned by "localtime()" is used.  *format* must be a string.
   "ValueError" is raised if any field in *t* is outside of the
   allowed range.

   0 is a legal argument for any position in the time tuple; if it is
   normally illegal the value is forced to a correct one.

   The following directives can be embedded in the *format* string.
   They are shown without the optional field width and precision
   specification, and are replaced by the indicated characters in the
   "strftime()" result:

   +-------------+--------------------------------------------------+---------+
   | Directive   | Meaning                                          | Notes   |
   +=============+==================================================+=========+
   | "%a"        | Locale’s abbreviated weekday name.               |         |
   +-------------+--------------------------------------------------+---------+
   | "%A"        | Locale’s full weekday name.                      |         |
   +-------------+--------------------------------------------------+---------+
   | "%b"        | Locale’s abbreviated month name.                 |         |
   +-------------+--------------------------------------------------+---------+
   | "%B"        | Locale’s full month name.                        |         |
   +-------------+--------------------------------------------------+---------+
   | "%c"        | Locale’s appropriate date and time               |         |
   |             | representation.                                  |         |
   +-------------+--------------------------------------------------+---------+
   | "%d"        | Day of the month as a decimal number [01,31].    |         |
   +-------------+--------------------------------------------------+---------+
   | "%H"        | Hour (24-hour clock) as a decimal number         |         |
   |             | [00,23].                                         |         |
   +-------------+--------------------------------------------------+---------+
   | "%I"        | Hour (12-hour clock) as a decimal number         |         |
   |             | [01,12].                                         |         |
   +-------------+--------------------------------------------------+---------+
   | "%j"        | Day of the year as a decimal number [001,366].   |         |
   +-------------+--------------------------------------------------+---------+
   | "%m"        | Month as a decimal number [01,12].               |         |
   +-------------+--------------------------------------------------+---------+
   | "%M"        | Minute as a decimal number [00,59].              |         |
   +-------------+--------------------------------------------------+---------+
   | "%p"        | Locale’s equivalent of either AM or PM.          | (1)     |
   +-------------+--------------------------------------------------+---------+
   | "%S"        | Second as a decimal number [00,61].              | (2)     |
   +-------------+--------------------------------------------------+---------+
   | "%U"        | Week number of the year (Sunday as the first day | (3)     |
   |             | of the week) as a decimal number [00,53]. All    |         |
   |             | days in a new year preceding the first Sunday    |         |
   |             | are considered to be in week 0.                  |         |
   +-------------+--------------------------------------------------+---------+
   | "%w"        | Weekday as a decimal number [0(Sunday),6].       |         |
   +-------------+--------------------------------------------------+---------+
   | "%W"        | Week number of the year (Monday as the first day | (3)     |
   |             | of the week) as a decimal number [00,53]. All    |         |
   |             | days in a new year preceding the first Monday    |         |
   |             | are considered to be in week 0.                  |         |
   +-------------+--------------------------------------------------+---------+
   | "%x"        | Locale’s appropriate date representation.        |         |
   +-------------+--------------------------------------------------+---------+
   | "%X"        | Locale’s appropriate time representation.        |         |
   +-------------+--------------------------------------------------+---------+
   | "%y"        | Year without century as a decimal number         |         |
   |             | [00,99].                                         |         |
   +-------------+--------------------------------------------------+---------+
   | "%Y"        | Year with century as a decimal number.           |         |
   +-------------+--------------------------------------------------+---------+
   | "%z"        | Time zone offset indicating a positive or        |         |
   |             | negative time difference from UTC/GMT of the     |         |
   |             | form +HHMM or -HHMM, where H represents decimal  |         |
   |             | hour digits and M represents decimal minute      |         |
   |             | digits [-23:59, +23:59].                         |         |
   +-------------+--------------------------------------------------+---------+
   | "%Z"        | Time zone name (no characters if no time zone    |         |
   |             | exists).                                         |         |
   +-------------+--------------------------------------------------+---------+
   | "%%"        | A literal "'%'" character.                       |         |
   +-------------+--------------------------------------------------+---------+

   Notes:

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

   2. The range really is "0" to "61"; value "60" is valid in
      timestamps representing leap seconds and value "61" is supported
      for historical reasons.

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

   Here is an example, a format for dates compatible with that
   specified  in the **RFC 2822** Internet email standard.  [1]

      >>> from time import gmtime, strftime
      >>> strftime("%a, %d %b %Y %H:%M:%S +0000", gmtime())
      'Thu, 28 Jun 2001 14:17:15 +0000'

   Additional directives may be supported on certain platforms, but
   only the ones listed here have a meaning standardized by ANSI C.
   To see the full set of format codes supported on your platform,
   consult the *strftime(3)* documentation.

   On some platforms, an optional field width and precision
   specification can immediately follow the initial "'%'" of a
   directive in the following order; this is also not portable. The
   field width is normally 2 except for "%j" where it is 3.

time.strptime(string[, format])

   Parse a string representing a time according to a format.  The
   return value is a "struct_time" as returned by "gmtime()" or
   "localtime()".

   The *format* parameter uses the same directives as those used by
   "strftime()"; it defaults to ""%a %b %d %H:%M:%S %Y"" which matches
   the formatting returned by "ctime()". If *string* cannot be parsed
   according to *format*, or if it has excess data after parsing,
   "ValueError" is raised. The default values used to fill in any
   missing data when more accurate values cannot be inferred are
   "(1900, 1, 1, 0, 0, 0, 0, 1, -1)". Both *string* and *format* must
   be strings.

   For example:

   >>> import time
   >>> time.strptime("30 Nov 00", "%d %b %y")   # doctest: +NORMALIZE_WHITESPACE
   time.struct_time(tm_year=2000, tm_mon=11, tm_mday=30, tm_hour=0, tm_min=0,
                    tm_sec=0, tm_wday=3, tm_yday=335, tm_isdst=-1)

   Support for the "%Z" directive is based on the values contained in
   "tzname" and whether "daylight" is true.  Because of this, it is
   platform-specific except for recognizing UTC and GMT which are
   always known (and are considered to be non-daylight savings
   timezones).

   Only the directives specified in the documentation are supported.
   Because "strftime()" is implemented per platform it can sometimes
   offer more directives than those listed.  But "strptime()" is
   independent of any platform and thus does not necessarily support
   all directives available that are not documented as supported.

class time.struct_time

      The type of the time value sequence returned by "gmtime()",
      "localtime()", and "strptime()".  It is an object with a *named
      tuple* interface: values can be accessed by index and by
      attribute name.  The following values are present:

      +---------+---------------------+-----------------------------------+
      | Index   | Attribute           | Values                            |
      +=========+=====================+===================================+
      | 0       | "tm_year"           | (for example, 1993)               |
      +---------+---------------------+-----------------------------------+
      | 1       | "tm_mon"            | range [1, 12]                     |
      +---------+---------------------+-----------------------------------+
      | 2       | "tm_mday"           | range [1, 31]                     |
      +---------+---------------------+-----------------------------------+
      | 3       | "tm_hour"           | range [0, 23]                     |
      +---------+---------------------+-----------------------------------+
      | 4       | "tm_min"            | range [0, 59]                     |
      +---------+---------------------+-----------------------------------+
      | 5       | "tm_sec"            | range [0, 61]; see **(2)** in     |
      |         |                     | "strftime()" description          |
      +---------+---------------------+-----------------------------------+
      | 6       | "tm_wday"           | range [0, 6], Monday is 0         |
      +---------+---------------------+-----------------------------------+
      | 7       | "tm_yday"           | range [1, 366]                    |
      +---------+---------------------+-----------------------------------+
      | 8       | "tm_isdst"          | 0, 1 or -1; see below             |
      +---------+---------------------+-----------------------------------+
      | N/A     | "tm_zone"           | abbreviation of timezone name     |
      +---------+---------------------+-----------------------------------+
      | N/A     | "tm_gmtoff"         | offset east of UTC in seconds     |
      +---------+---------------------+-----------------------------------+

      Note that unlike the C structure, the month value is a range of
      [1, 12], not [0, 11].

      In calls to "mktime()", "tm_isdst" may be set to 1 when daylight
      savings time is in effect, and 0 when it is not.  A value of -1
      indicates that this is not known, and will usually result in the
      correct state being filled in.

      When a tuple with an incorrect length is passed to a function
      expecting a "struct_time", or having elements of the wrong type,
      a "TypeError" is raised.

   Changed in version 3.3: "tm_gmtoff" and "tm_zone" attributes are
   available on platforms with C library supporting the corresponding
   fields in "struct tm".

time.time()

   Return the time in seconds since the epoch as a floating point
   number. The specific date of the epoch and the handling of leap
   seconds is platform dependent. On Windows and most Unix systems,
   the epoch is January 1, 1970, 00:00:00 (UTC) and leap seconds are
   not counted towards the time in seconds since the epoch. This is
   commonly referred to as Unix time. To find out what the epoch is on
   a given platform, look at "gmtime(0)".

   Note that even though the time is always returned as a floating
   point number, not all systems provide time with a better precision
   than 1 second. While this function normally returns non-decreasing
   values, it can return a lower value than a previous call if the
   system clock has been set back between the two calls.

   The number returned by "time()" may be converted into a more common
   time format (i.e. year, month, day, hour, etc…) in UTC by passing
   it to "gmtime()" function or in local time by passing it to the
   "localtime()" function. In both cases a "struct_time" object is
   returned, from which the components of the calendar date may be
   accessed as attributes.

time.timezone

   The offset of the local (non-DST) timezone, in seconds west of UTC
   (negative in most of Western Europe, positive in the US, zero in
   the UK).

time.tzname

   A tuple of two strings: the first is the name of the local non-DST
   timezone, the second is the name of the local DST timezone.  If no
   DST timezone is defined, the second string should not be used.

time.tzset()

   Resets the time conversion rules used by the library routines. The
   environment variable "TZ" specifies how this is done.

   Availability: Unix.

   Note: Although in many cases, changing the "TZ" environment
     variable may affect the output of functions like "localtime()"
     without calling "tzset()", this behavior should not be relied
     on.The "TZ" environment variable should contain no whitespace.

   The standard format of the "TZ" environment variable is (whitespace
   added for clarity):

      std offset [dst [offset [,start[/time], end[/time]]]]

   Where the components are:

   "std" and "dst"
      Three or more alphanumerics giving the timezone abbreviations.
      These will be propagated into time.tzname

   "offset"
      The offset has the form: "± hh[:mm[:ss]]". This indicates the
      value added the local time to arrive at UTC.  If preceded by a
      ‘-‘, the timezone is east of the Prime Meridian; otherwise, it
      is west. If no offset follows dst, summer time is assumed to be
      one hour ahead of standard time.

   "start[/time], end[/time]"
      Indicates when to change to and back from DST. The format of the
      start and end dates are one of the following:

      "J*n*"
         The Julian day *n* (1 <= *n* <= 365). Leap days are not
         counted, so in all years February 28 is day 59 and March 1 is
         day 60.

      "*n*"
         The zero-based Julian day (0 <= *n* <= 365). Leap days are
         counted, and it is possible to refer to February 29.

      "M*m*.*n*.*d*"
         The *d*’th day (0 <= *d* <= 6) or week *n* of month *m* of
         the year (1 <= *n* <= 5, 1 <= *m* <= 12, where week 5 means
         “the last *d* day in month *m*” which may occur in either the
         fourth or the fifth week). Week 1 is the first week in which
         the *d*’th day occurs. Day zero is Sunday.

      "time" has the same format as "offset" except that no leading
      sign (‘-‘ or ‘+’) is allowed. The default, if time is not given,
      is 02:00:00.

      >>> os.environ['TZ'] = 'EST+05EDT,M4.1.0,M10.5.0'
      >>> time.tzset()
      >>> time.strftime('%X %x %Z')
      '02:07:36 05/08/03 EDT'
      >>> os.environ['TZ'] = 'AEST-10AEDT-11,M10.5.0,M3.5.0'
      >>> time.tzset()
      >>> time.strftime('%X %x %Z')
      '16:08:12 05/08/03 AEST'

   On many Unix systems (including *BSD, Linux, Solaris, and Darwin),
   it is more convenient to use the system’s zoneinfo (*tzfile(5)*)
   database to specify the timezone rules. To do this, set the  "TZ"
   environment variable to the path of the required timezone
   datafile, relative to the root of the systems ‘zoneinfo’ timezone
   database, usually located at "/usr/share/zoneinfo". For example,
   "'US/Eastern'", "'Australia/Melbourne'", "'Egypt'" or
   "'Europe/Amsterdam'".

      >>> os.environ['TZ'] = 'US/Eastern'
      >>> time.tzset()
      >>> time.tzname
      ('EST', 'EDT')
      >>> os.environ['TZ'] = 'Egypt'
      >>> time.tzset()
      >>> time.tzname
      ('EET', 'EEST')

See also:

  Module "datetime"
     More object-oriented interface to dates and times.

  Module "locale"
     Internationalization services.  The locale setting affects the
     interpretation of many format specifiers in "strftime()" and
     "strptime()".

  Module "calendar"
     General calendar-related functions.   "timegm()" is the inverse
     of "gmtime()" from this module.

-[ Footnotes ]-

[1] The use of "%Z" is now deprecated, but the "%z" escape that
    expands to the preferred  hour/minute offset is not supported by
    all ANSI C libraries. Also, a strict reading of the original 1982
    **RFC 822** standard calls for a two-digit year (%y rather than
    %Y), but practice moved to 4-digit years long before the year
    2000.  After that, **RFC 822** became obsolete and the 4-digit
    year has been first recommended by **RFC 1123** and then mandated
    by **RFC 2822**.
