15.3. "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.  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.

* 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()", may be considered as 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 2.2: The time value sequence was changed from a
  tuple to a "struct_time", with the addition of attribute names for
  the fields.

* 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.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.

   Changed in version 2.1: Allowed *t* to be omitted.

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()".

   Changed in version 2.1: Allowed *secs* to be omitted.

   Changed in version 2.4: If *secs* is "None", the current time is
   used.

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.

   Changed in version 2.1: Allowed *secs* to be omitted.

   Changed in version 2.4: If *secs* is "None", the current time is
   used.

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.

   Changed in version 2.1: Allowed *secs* to be omitted.

   Changed in version 2.4: If *secs* is "None", the current time is
   used.

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 of the current 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.

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. "strftime()" returns a locale dependent byte string;
   the result may be converted to unicode by doing
   "strftime(<myformat>).decode(locale.getlocale()[1])".

   Changed in version 2.1: Allowed *t* to be omitted.

   Changed in version 2.4: "ValueError" raised if a field in *t* is
   out of range.

   Changed in version 2.5: 0 is now 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 | (1)     |
   |             | or PM.                           |         |
   +-------------+----------------------------------+---------+
   | "%S"        | Second as a decimal number       | (2)     |
   |             | [00,61].                         |         |
   +-------------+----------------------------------+---------+
   | "%U"        | Week number of the year (Sunday  | (3)     |
   |             | as the first day 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  | (3)     |
   |             | as the first day 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.
   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)".

   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             |
   +---------+---------------------+-----------------------------------+

   New in version 2.2.

   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.

   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.

time.time()

   Return the time in seconds since the epoch as a floating point
   number. 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()

   Reset the time conversion rules used by the library routines. The
   environment variable "TZ" specifies how this is done. It will also
   set the variables "tzname" (from the "TZ" environment variable),
   "timezone" (non-DST seconds West of UTC), "altzone" (DST seconds
   west of UTC) and "daylight" (to 0 if this timezone does not have
   any daylight saving time rules, or to nonzero if there is a time,
   past, present or future when daylight saving time applies).

   New in version 2.3.

   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**.
