
``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.  On January 1st of
  that year, at 0 hours, the "time since the epoch" is zero.  For
  Unix, the epoch is 1970.  To find out what the epoch is, look at
  ``gmtime(0)``.

* The functions in this module do 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 Unix, 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.  Functions accepting a ``struct_time`` (see below) generally
  require a 4-digit year.  For backward compatibility, 2-digit years
  are supported if the module variable ``accept2dyear`` is a non-zero
  integer; this variable is initialized to ``1`` unless the
  environment variable **PYTHONY2K** is set to a non-empty string, in
  which case it is initialized to ``0``.  Thus, you can set
  **PYTHONY2K** to a non-empty string in the environment to require
  4-digit years for all year input.  When 2-digit years are accepted,
  they are converted according to the POSIX or X/Open standard: values
  69-99 are mapped to 1969-1999, and values 0--68 are mapped to 2000--
  2068. Values 100--1899 are always illegal. Note that this is new as
  of Python 1.5.2(a2); earlier versions, up to Python 1.5.1 and
  1.5.2a1, would add 1900 to year values below 1900.

* 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.

  +---------+--------------------+--------------------------------+
  | 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 **(1)** 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          |
  +---------+--------------------+--------------------------------+

  Note that unlike the C structure, the month value is a range of
  1-12, not 0-11. A year value will be handled as described under
  "Year 2000 (Y2K) issues" above. A ``-1`` argument as the daylight
  savings flag, passed to ``mktime()`` will usually result in the
  correct daylight savings state to be 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.

The module defines the following functions and data items:

time.accept2dyear

   Boolean value indicating whether two-digit year values will be
   accepted.  This is true by default, but will be set to false if the
   environment variable **PYTHONY2K** has been set to a non-empty
   string.  It may also be modified at run time.

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 24-character 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, there is no 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, but in any case, this is the
   function to use for benchmarking Python or timing algorithms.

   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.

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.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.sleep(secs)

   Suspend execution 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.

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 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``; this accounts for leap
      seconds and the (very rare) double leap seconds.

   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.

   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)``.

   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.

time.struct_time

   The type of the time value sequence returned by ``gmtime()``,
   ``localtime()``, and ``strptime()``.

time.time()

   Return the time as a floating point number expressed in seconds
   since the epoch, in UTC.  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.

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 settings can affect
      the return values for some of  the functions in the ``time``
      module.

   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.  The 4-digit year has been mandated by **RFC 2822**, which
    obsoletes **RFC 822**.
