# 6.2 Built-in Module `time`

This module provides various time-related functions. It is always available.

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)`.
- UTC is Coordinated Universal Time (formerly known as Greenwich Mean Time). 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, and on the Mac, times are only accurate to whole seconds.
- 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 tuple as returned by `gmtime()` and `localtime()`, or as accpted by `mktime()` is a tuple of 9 integers: year (e.g. 1993), month (1-12), day (1-31), hour (0-23), minute (0-59), second (0-59), weekday (0-6, monday is 0), Julian day (1-366) and daylight savings flag (-1, 0 or 1). Note that unlike the C structure, the month value is a range of 1-12, not 0-11. A year value less than 100 will typically be silently converted to 1900 plus the year value. A -1 argument as daylight savings flag, passed to `mktime()` will usually result in the correct daylight savings state to be filled in.

The module defines the following functions and data items:

`altzone`
  The offset of the local DST timezone, in seconds west of the 0th meridian, if one is defined. Negative if the local DST timezone is east of the 0th meridian (as in Western Europe, including the UK). Only use this if `daylight` is nonzero.

`asctime` (tuple)
  Convert a tuple 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'`. Note: unlike the C function of the same name, there is no trailing newline.

`clock` ()
  Return the current CPU time as a floating point number expressed in seconds. The precision, and in fact the very definiton of the meaning of ``CPU 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.

`ctime` (secs)
  Convert a time expressed in seconds since the epoch to a string representing local time. `ctime(t)` is equivalent to `asctime(localtime(t))`.

`daylight`
  Nonzero if a DST timezone is defined.

`gmtime` (secs)
  Convert a time expressed in seconds since the epoch to a time tuple in UTC in which the dst flag is always zero. Fractions of a second are ignored.

`localtime` (secs)
  Like `gmtime` but converts to local time. The dst flag is set to 1 when DST applies to the given time.

`mktime` (tuple)
  This is the inverse function of `localtime`. Its argument is the full 9-tuple (since the dst flag is needed -- pass -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.time()`. If the input value can't be represented as a valid time, OverflowError is raised.

`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.

`strftime` (format, tuple)
  Convert a tuple representing a time as returned by `gmtime()` or `localtime()` to a string as specified by the format argument.

  The following directives, shown without the optional field width and precision specification, are replaced by the indicated characters:

  Directive | Meaning
  --- | ---
  `%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.
  `%S` | Second as a decimal number [00,61].
  `%U` | Week number of the year (Sunday 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 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.
  `%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 (or by no characters if no time zone exists).

  `%%` | %

  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` ()
  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.

`timezone`
  The offset of the local (non-DST) timezone, in seconds west of the 0th meridian (i.e. negative in most of Western Europe, positive in the US, zero in the UK).

`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.