"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, the return value of
  "time.gmtime(0)". It is January 1, 1970, 00:00:00 (UTC) on all
  platforms.

* 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.

* 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* UTC is Coordinated Universal Time and superseded Greenwich Mean Time
  or GMT as the basis of international timekeeping. The acronym UTC is
  not a mistake but conforms to an earlier, language-agnostic naming
  scheme for time standards such as UT0, UT1, and UT2.

* 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.

  Changed in version 3.6: The "struct_time" attributes "tm_gmtoff" and
  "tm_zone" are now available on all platforms.

* 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                      |                           |                           |
  +---------------------------+---------------------------+---------------------------+


Functions
=========

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'". The day field is two characters long
   and is space padded if the day is a single digit, e.g.: "'Wed Jun
   9 04:26:40 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.pthread_getcpuclockid(thread_id)

   Return the *clk_id* of the thread-specific CPU-time clock for the
   specified *thread_id*.

   Use "threading.get_ident()" or the "ident" attribute of
   "threading.Thread" objects to get a suitable value for *thread_id*.

   Warning:

     Passing an invalid or expired *thread_id* may result in undefined
     behavior, such as segmentation fault.

   Availability: Unix

   See the man page for *pthread_getcpuclockid(3)* for further
   information.

   Added in version 3.7.

time.clock_getres(clk_id)

   Return the resolution (precision) of the specified clock *clk_id*.
   Refer to Clock ID Constants for a list of accepted values for
   *clk_id*.

   Availability: Unix.

   Added in version 3.3.

time.clock_gettime(clk_id) -> float

   Return the time of the specified clock *clk_id*.  Refer to Clock ID
   Constants for a list of accepted values for *clk_id*.

   Use "clock_gettime_ns()" to avoid the precision loss caused by the
   "float" type.

   Availability: Unix.

   Added in version 3.3.

time.clock_gettime_ns(clk_id) -> int

   Similar to "clock_gettime()" but return time as nanoseconds.

   Availability: Unix.

   Added in version 3.7.

time.clock_settime(clk_id, time: float)

   Set the time of the specified clock *clk_id*.  Currently,
   "CLOCK_REALTIME" is the only accepted value for *clk_id*.

   Use "clock_settime_ns()" to avoid the precision loss caused by the
   "float" type.

   Availability: Unix, not Android, not iOS.

   Added in version 3.3.

time.clock_settime_ns(clk_id, time: int)

   Similar to "clock_settime()" but set time with nanoseconds.

   Availability: Unix, not Android, not iOS.

   Added in version 3.7.

time.ctime([secs])

   Convert a time expressed in seconds since the epoch to a string of
   a form: "'Sun Jun 20 23:21:05 1993'" representing local time. The
   day field is two characters long and is space padded if the day is
   a single digit, e.g.: "'Wed Jun  9 04:26:40 1993'".

   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.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:

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

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

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

   * "'thread_time'": "time.thread_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.  Refer to Clock ID Constants for possible
     values.

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

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

   Added 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.

   "localtime()" may raise "OverflowError", if the timestamp is
   outside the range of values supported by the platform C
   "localtime()" or "gmtime()" functions, and "OSError" on
   "localtime()" or "gmtime()" failure. It’s common for this to be
   restricted to years between 1970 and 2038.

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() -> float

   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 two
   calls is valid.

   Clock:

   * On Windows, call "QueryPerformanceCounter()" and
     "QueryPerformanceFrequency()".

   * On macOS, call "mach_absolute_time()" and "mach_timebase_info()".

   * On HP-UX, call "gethrtime()".

   * Call "clock_gettime(CLOCK_HIGHRES)" if available.

   * Otherwise, call "clock_gettime(CLOCK_MONOTONIC)".

   Use "monotonic_ns()" to avoid the precision loss caused by the
   "float" type.

   Added in version 3.3.

   Changed in version 3.5: The function is now always available and
   the clock is now the same for all processes.

   Changed in version 3.10: On macOS, the clock is now the same for
   all processes.

time.monotonic_ns() -> int

   Similar to "monotonic()", but return time as nanoseconds.

   Added in version 3.7.

time.perf_counter() -> float

   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. The
   clock is the same for all processes. The reference point of the
   returned value is undefined, so that only the difference between
   the results of two calls is valid.

   **CPython implementation detail:** On CPython, use the same clock
   as "time.monotonic()" and is a monotonic clock, i.e. a clock that
   cannot go backwards.

   Use "perf_counter_ns()" to avoid the precision loss caused by the
   "float" type.

   Added in version 3.3.

   Changed in version 3.10: On Windows, the clock is now the same for
   all processes.

   Changed in version 3.13: Use the same clock as "time.monotonic()".

time.perf_counter_ns() -> int

   Similar to "perf_counter()", but return time as nanoseconds.

   Added in version 3.7.

time.process_time() -> float

   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 two calls is valid.

   Use "process_time_ns()" to avoid the precision loss caused by the
   "float" type.

   Added in version 3.3.

time.process_time_ns() -> int

   Similar to "process_time()" but return time as nanoseconds.

   Added in version 3.7.

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.

   If the sleep is interrupted by a signal and no exception is raised
   by the signal handler, the sleep is restarted with a recomputed
   timeout.

   The suspension time may be longer than requested by an arbitrary
   amount, because of the scheduling of other activity in the system.

   -[ Windows implementation ]-

   On Windows, if *secs* is zero, the thread relinquishes the
   remainder of its time slice to any other thread that is ready to
   run. If there are no other threads ready to run, the function
   returns immediately, and the thread continues execution.  On
   Windows 8.1 and newer the implementation uses a high-resolution
   timer which provides resolution of 100 nanoseconds. If *secs* is
   zero, "Sleep(0)" is used.

   -[ Unix implementation ]-

   * Use "clock_nanosleep()" if available (resolution: 1 nanosecond);

   * Or use "nanosleep()" if available (resolution: 1 nanosecond);

   * Or use "select()" (resolution: 1 microsecond).

   Note:

     To emulate a “no-op”, use "pass" instead of "time.sleep(0)".To
     voluntarily relinquish the CPU, specify a real-time scheduling
     policy and use "os.sched_yield()" instead.

   Raises an auditing event "time.sleep" with argument "secs".

   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).

   Changed in version 3.11: On Unix, the "clock_nanosleep()" and
   "nanosleep()" functions are now used if available. On Windows, a
   waitable timer is now used.

   Changed in version 3.13: Raises an auditing event.

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].    |         |
   +-------------+--------------------------------------------------+---------+
   | "%f"        | Microseconds as a decimal number                 | (1)     |
   |             | [000000,999999].                                 |         |
   +-------------+--------------------------------------------------+---------+
   | "%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.          | (2)     |
   +-------------+--------------------------------------------------+---------+
   | "%S"        | Second as a decimal number [00,61].              | (3)     |
   +-------------+--------------------------------------------------+---------+
   | "%U"        | Week number of the year (Sunday as the first day | (4)     |
   |             | 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.                  |         |
   +-------------+--------------------------------------------------+---------+
   | "%u"        | Day of the week (Monday is 1; Sunday is 7) as a  |         |
   |             | decimal number [1, 7].                           |         |
   +-------------+--------------------------------------------------+---------+
   | "%w"        | Weekday as a decimal number [0(Sunday),6].       |         |
   +-------------+--------------------------------------------------+---------+
   | "%W"        | Week number of the year (Monday as the first day | (4)     |
   |             | 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]. [1]                     |         |
   +-------------+--------------------------------------------------+---------+
   | "%Z"        | Time zone name (no characters if no time zone    |         |
   |             | exists). Deprecated. [1]                         |         |
   +-------------+--------------------------------------------------+---------+
   | "%G"        | ISO 8601 year (similar to "%Y" but follows the   |         |
   |             | rules for the ISO 8601 calendar year). The year  |         |
   |             | starts with the week that contains the first     |         |
   |             | Thursday of the calendar year.                   |         |
   +-------------+--------------------------------------------------+---------+
   | "%V"        | ISO 8601 week number (as a decimal number        |         |
   |             | [01,53]). The first week of the year is the one  |         |
   |             | that contains the first Thursday of the year.    |         |
   |             | Weeks start on Monday.                           |         |
   +-------------+--------------------------------------------------+---------+
   | "%%"        | A literal "'%'" character.                       |         |
   +-------------+--------------------------------------------------+---------+

   Notes:

   1. The "%f" format directive only applies to "strptime()", not to
      "strftime()". However, see also "datetime.datetime.strptime()"
      and "datetime.datetime.strftime()" where the "%f" format
      directive applies to microseconds.

   2. 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.

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

   4. 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")
   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 Note (2) in    |
   |                                   |                                   | "strftime()"                      |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | 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.

time.time() -> float

   Return the time in seconds since the epoch as a floating-point
   number. The handling of leap seconds is platform dependent. On
   Windows and most Unix systems, the leap seconds are not counted
   towards the time in seconds since the epoch. This is commonly
   referred to as Unix time.

   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.

   Clock:

   * On Windows, call "GetSystemTimePreciseAsFileTime()".

   * Call "clock_gettime(CLOCK_REALTIME)" if available.

   * Otherwise, call "gettimeofday()".

   Use "time_ns()" to avoid the precision loss caused by the "float"
   type.

Changed in version 3.13: On Windows, calls
"GetSystemTimePreciseAsFileTime()" instead of
"GetSystemTimeAsFileTime()".

time.time_ns() -> int

   Similar to "time()" but returns time as an integer number of
   nanoseconds since the epoch.

   Added in version 3.7.

time.thread_time() -> float

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

   Use "thread_time_ns()" to avoid the precision loss caused by the
   "float" type.

   Availability: Linux, Unix, Windows.

   Unix systems supporting "CLOCK_THREAD_CPUTIME_ID".

   Added in version 3.7.

time.thread_time_ns() -> int

   Similar to "thread_time()" but return time as nanoseconds.

   Added in version 3.7.

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).

   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) of 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 a 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')


Clock ID Constants
==================

These constants are used as parameters for "clock_getres()" and
"clock_gettime()".

time.CLOCK_BOOTTIME

   Identical to "CLOCK_MONOTONIC", except it also includes any time
   that the system is suspended.

   This allows applications to get a suspend-aware monotonic  clock
   without having to deal with the complications of "CLOCK_REALTIME",
   which may have  discontinuities if the time is changed using
   "settimeofday()" or similar.

   Availability: Linux >= 2.6.39.

   Added in version 3.7.

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.

   Added in version 3.3.

time.CLOCK_MONOTONIC

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

   Availability: Unix.

   Added 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, macOS >= 10.12.

   Added in version 3.3.

time.CLOCK_MONOTONIC_RAW_APPROX

   Similar to "CLOCK_MONOTONIC_RAW", but reads a value cached by the
   system at context switch and hence has less accuracy.

   Availability: macOS >= 10.12.

   Added in version 3.13.

time.CLOCK_PROCESS_CPUTIME_ID

   High-resolution per-process timer from the CPU.

   Availability: Unix.

   Added in version 3.3.

time.CLOCK_PROF

   High-resolution per-process timer from the CPU.

   Availability: FreeBSD, NetBSD >= 7, OpenBSD.

   Added in version 3.7.

time.CLOCK_TAI

   International Atomic Time

   The system must have a current leap second table in order for this
   to give the correct answer.  PTP or NTP software can maintain a
   leap second table.

   Availability: Linux.

   Added in version 3.9.

time.CLOCK_THREAD_CPUTIME_ID

   Thread-specific CPU-time clock.

   Availability: Unix.

   Added in version 3.3.

time.CLOCK_UPTIME

   Time whose absolute value is the time the system has been running
   and not suspended, providing accurate uptime measurement, both
   absolute and interval.

   Availability: FreeBSD, OpenBSD >= 5.5.

   Added in version 3.7.

time.CLOCK_UPTIME_RAW

   Clock that increments monotonically, tracking the time since an
   arbitrary point, unaffected by frequency or time adjustments and
   not incremented while the system is asleep.

   Availability: macOS >= 10.12.

   Added in version 3.8.

time.CLOCK_UPTIME_RAW_APPROX

   Like "CLOCK_UPTIME_RAW", but the value is cached by the system at
   context switches and therefore has less accuracy.

   Availability: macOS >= 10.12.

   Added in version 3.13.

The following constant is the only parameter that can be sent to
"clock_settime()".

time.CLOCK_REALTIME

   Real-time clock.  Setting this clock requires appropriate
   privileges. The clock is the same for all processes.

   Availability: Unix.

   Added in version 3.3.


Timezone Constants
==================

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.  See note below.

time.daylight

   Nonzero if a DST timezone is defined.  See note below.

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).  See note below.

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.  See
   note below.

Note:

  For the above Timezone constants ("altzone", "daylight", "timezone",
  and "tzname"), the value is determined by the timezone rules in
  effect at module load time or the last time "tzset()" is called and
  may be incorrect for times in the past.  It is recommended to use
  the "tm_gmtoff" and "tm_zone" results from "localtime()" to obtain
  timezone information.

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**.
