Doc/libtime.tex

   1 \section{Built-in Module \sectcode{time}}
   2
   3 \bimodindex{time}
   4 This module provides various time-related functions.
   5 It is always available.
   6
   7 An explanation of some terminology and conventions is in order.
   8
   9 \begin{itemize}
  10
  11 \item
  12 The ``epoch'' is the point where the time starts.  On January 1st of that
  13 year, at 0 hours, the ``time since the epoch'' is zero.  For UNIX, the
  14 epoch is 1970.  To find out what the epoch is, look at \code{gmtime(0)}.
  15
  16 \item
  17 UTC is Coordinated Universal Time (formerly known as Greenwich Mean
  18 Time).  The acronym UTC is not a mistake but a compromise between
  19 English and French.
  20
  21 \item
  22 DST is Daylight Saving Time, an adjustment of the timezone by
  23 (usually) one hour during part of the year.  DST rules are magic
  24 (determined by local law) and can change from year to year.  The C
  25 library has a table containing the local rules (often it is read from
  26 a system file for flexibility) and is the only source of True Wisdom
  27 in this respect.
  28
  29 \item
  30 The precision of the various real-time functions may be less than
  31 suggested by the units in which their value or argument is expressed.
  32 E.g.\ on most UNIX systems, the clock ``ticks'' only 50 or 100 times a
  33 second, and on the Mac, times are only accurate to whole seconds.
  34
  35 \end{itemize}
  36
  37 The module defines the following functions and data items:
  38
  39 \renewcommand{\indexsubitem}{(in module time)}
  40
  41 \begin{datadesc}{altzone}
  42 The offset of the local DST timezone, in seconds west of the 0th
  43 meridian, if one is defined.  Negative if the local DST timezone is
  44 east of the 0th meridian (as in Western Europe, including the UK).
  45 Only use this if \code{daylight} is nonzero.
  46 \end{datadesc}
  47
  48
  49 \begin{funcdesc}{asctime}{tuple}
  50 Convert a tuple representing a time as returned by \code{gmtime()} or
  51 \code{localtime()} to a 24-character string of the following form:
  52 \code{'Sun Jun 20 23:21:05 1993'}.  Note: unlike the C function of
  53 the same name, there is no trailing newline.
  54 \end{funcdesc}
  55
  56
  57 \begin{funcdesc}{clock}{}
  58 Return the current CPU time as a floating point number expressed in
  59 seconds.  The precision, and in fact the very definiton of the meaning
  60 of ``CPU time'', depends on that of the C function of the same name.
  61 \end{funcdesc}
  62
  63
  64 \begin{funcdesc}{ctime}{secs}
  65 Convert a time expressed in seconds since the epoch to a string
  66 representing local time.  \code{ctime(t)} is equivalent to
  67 \code{asctime(localtime(t))}.
  68 \end{funcdesc}
  69
  70 \begin{datadesc}{daylight}
  71 Nonzero if a DST timezone is defined.
  72 \end{datadesc}
  73
  74 \begin{funcdesc}{gmtime}{secs}
  75 Convert a time expressed in seconds since the epoch to a tuple of 9
  76 integers, in UTC: year (e.g.\ 1993), month (1--12), day (1--31), hour
  77 (0--23), minute (0--59), second (0--59), weekday (0--6, monday is 0),
  78 Julian day (1--366), dst flag (always zero).  Fractions of a second are
  79 ignored.  Note subtle differences with the C function of this name.
  80 \end{funcdesc}
  81
  82 \begin{funcdesc}{localtime}{secs}
  83 Like \code{gmtime} but converts to local time.  The dst flag is set
  84 to 1 when DST applies to the given time.
  85 \end{funcdesc}
  86
  87 \begin{funcdesc}{mktime}{tuple}
  88 This is the inverse function of \code{localtime}.  Its argument is the
  89 full 9-tuple (since the dst flag is needed).  It returns an integer.
  90 \end{funcdesc}
  91
  92 \begin{funcdesc}{sleep}{secs}
  93 Suspend execution for the given number of seconds.  The argument may
  94 be a floating point number to indicate a more precise sleep time.
  95 \end{funcdesc}
  96
  97 \begin{funcdesc}{time}{}
  98 Return the time as a floating point number expressed in seconds since
  99 the epoch, in UTC.  Note that even though the time is always returned
 100 as a floating point number, not all systems provide time with a better
 101 precision than 1 second.
 102 \end{funcdesc}
 103
 104 \begin{datadesc}{timezone}
 105 The offset of the local (non-DST) timezone, in seconds west of the 0th
 106 meridian (i.e. negative in most of Western Europe, positive in the US,
 107 zero in the UK).
 108 \end{datadesc}
 109
 110 \begin{datadesc}{tzname}
 111 A tuple of two strings: the first is the name of the local non-DST
 112 timezone, the second is the name of the local DST timezone.  If no DST
 113 timezone is defined, the second string should not be used.
 114 \end{datadesc}