path: root/Doc/lib/libtime.tex

\section{\module{time} ---
         Time access and conversions}

\declaremodule{builtin}{time}
\modulesynopsis{Time access and conversions.}


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

\begin{itemize}

\item
The \dfn{epoch}\index{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 \code{gmtime(0)}.

\item
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\index{Year 2038}.

\item
\strong{Year 2000 (Y2K) issues}:\index{Year 2000}\index{Y2K}  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 \class{struct_time}
(see below) generally require a 4-digit year.  For backward
compatibility, 2-digit years are supported if the module variable
\code{accept2dyear} is a non-zero integer; this variable is
initialized to \code{1} unless the environment variable
\envvar{PYTHONY2K} is set to a non-empty string, in which case it is
initialized to \code{0}.  Thus, you can set
\envvar{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.

\item
UTC\index{UTC} is Coordinated Universal Time\index{Coordinated
Universal Time} (formerly known as Greenwich Mean
Time,\index{Greenwich Mean Time} or GMT).  The acronym UTC is not a
mistake but a compromise between English and French.

\item
DST is Daylight Saving Time,\index{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.

\item
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.

\item
On the other hand, the precision of \function{time()} and
\function{sleep()} is better than their \UNIX{} equivalents: times are
expressed as floating point numbers, \function{time()} returns the
most accurate time available (using \UNIX{} \cfunction{gettimeofday()}
where available), and \function{sleep()} will accept a time with a
nonzero fraction (\UNIX{} \cfunction{select()} is used to implement
this, where available).

\item
The time value as returned by \function{gmtime()},
\function{localtime()}, and \function{strptime()}, and accepted by
\function{asctime()}, \function{mktime()} and \function{strftime()},
is a sequence of 9 integers.  The return values of \function{gmtime()},
\function{localtime()}, and \function{strptime()} also offer attribute
names for individual fields.

\begin{tableiii}{c|l|l}{textrm}{Index}{Attribute}{Values}
  \lineiii{0}{\member{tm_year}}{(for example, 1993)}
  \lineiii{1}{\member{tm_mon}}{range [1,12]}
  \lineiii{2}{\member{tm_mday}}{range [1,31]}
  \lineiii{3}{\member{tm_hour}}{range [0,23]}
  \lineiii{4}{\member{tm_min}}{range [0,59]}
  \lineiii{5}{\member{tm_sec}}{range [0,61]; see \strong{(1)} in \function{strftime()} description}
  \lineiii{6}{\member{tm_wday}}{range [0,6], Monday is 0}
  \lineiii{7}{\member{tm_yday}}{range [1,366]}
  \lineiii{8}{\member{tm_isdst}}{0, 1 or -1; see below}
\end{tableiii}

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 \code{-1} argument as the
daylight savings flag, passed to \function{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 \class{struct_time}, or having elements of the wrong type, a
\exception{TypeError} is raised.

\versionchanged[The time value sequence was changed from a tuple to a
                \class{struct_time}, with the addition of attribute names
                for the fields]{2.2}
\end{itemize}

The module defines the following functions and data items:


\begin{datadesc}{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 \envvar{PYTHONY2K} has been set to a non-empty
string.  It may also be modified at run time.
\end{datadesc}

\begin{datadesc}{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
\code{daylight} is nonzero.
\end{datadesc}

\begin{funcdesc}{asctime}{\optional{t}}
Convert a tuple or \class{struct_time} representing a time as returned
by \function{gmtime()}
or \function{localtime()} to a 24-character string of the following form:
\code{'Sun Jun 20 23:21:05 1993'}.  If \var{t} is not provided, the
current time as returned by \function{localtime()} is used.
Locale information is not used by \function{asctime()}.
\note{Unlike the C function of the same name, there is no trailing
newline.}
\versionchanged[Allowed \var{t} to be omitted]{2.1}
\end{funcdesc}

\begin{funcdesc}{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''\index{CPU time}\index{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\index{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 \cfunction{QueryPerformanceCounter()}.
The resolution is typically better than one microsecond.
\end{funcdesc}

\begin{funcdesc}{ctime}{\optional{secs}}
Convert a time expressed in seconds since the epoch to a string
representing local time. If \var{secs} is not provided, the current time
as returned by \function{time()} is used.  \code{ctime(\var{secs})}
is equivalent to \code{asctime(localtime(\var{secs}))}.
Locale information is not used by \function{ctime()}.
\versionchanged[Allowed \var{secs} to be omitted]{2.1}
\end{funcdesc}

\begin{datadesc}{daylight}
Nonzero if a DST timezone is defined.
\end{datadesc}

\begin{funcdesc}{gmtime}{\optional{secs}}
Convert a time expressed in seconds since the epoch to a \class{struct_time}
in UTC in which the dst flag is always zero.  If \var{secs} is not
provided, the current time as returned by \function{time()} is used.
Fractions of a second are ignored.  See above for a description of the
\class{struct_time} object.
\versionchanged[Allowed \var{secs} to be omitted]{2.1}
\end{funcdesc}

\begin{funcdesc}{localtime}{\optional{secs}}
Like \function{gmtime()} but converts to local time.  The dst flag is
set to \code{1} when DST applies to the given time.
\versionchanged[Allowed \var{secs} to be omitted]{2.1}
\end{funcdesc}

\begin{funcdesc}{mktime}{t}
This is the inverse function of \function{localtime()}.  Its argument
is the \class{struct_time} or full 9-tuple (since the dst flag is
needed; use \code{-1} as the dst flag if it is unknown) which
expresses the time in
\emph{local} time, not UTC.  It returns a floating point number, for
compatibility with \function{time()}.  If the input value cannot be
represented as a valid time, either \exception{OverflowError} or
\exception{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.
\end{funcdesc}

\begin{funcdesc}{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 \function{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.
\end{funcdesc}

\begin{funcdesc}{strftime}{format\optional{, t}}
Convert a tuple or \class{struct_time} representing a time as returned
by \function{gmtime()} or \function{localtime()} to a string as
specified by the \var{format} argument.  If \var{t} is not
provided, the current time as returned by \function{localtime()} is
used.  \var{format} must be a string.
\versionchanged[Allowed \var{t} to be omitted]{2.1}

The following directives can be embedded in the \var{format} string.
They are shown without the optional field width and precision
specification, and are replaced by the indicated characters in the
\function{strftime()} result:

\begin{tableiii}{c|p{24em}|c}{code}{Directive}{Meaning}{Notes}
  \lineiii{\%a}{Locale's abbreviated weekday name.}{}
  \lineiii{\%A}{Locale's full weekday name.}{}
  \lineiii{\%b}{Locale's abbreviated month name.}{}
  \lineiii{\%B}{Locale's full month name.}{}
  \lineiii{\%c}{Locale's appropriate date and time representation.}{}
  \lineiii{\%d}{Day of the month as a decimal number [01,31].}{}
  \lineiii{\%H}{Hour (24-hour clock) as a decimal number [00,23].}{}
  \lineiii{\%I}{Hour (12-hour clock) as a decimal number [01,12].}{}
  \lineiii{\%j}{Day of the year as a decimal number [001,366].}{}
  \lineiii{\%m}{Month as a decimal number [01,12].}{}
  \lineiii{\%M}{Minute as a decimal number [00,59].}{}
  \lineiii{\%p}{Locale's equivalent of either AM or PM.}{}
  \lineiii{\%S}{Second as a decimal number [00,61].}{(1)}
  \lineiii{\%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.}{}
  \lineiii{\%w}{Weekday as a decimal number [0(Sunday),6].}{}
  \lineiii{\%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 Monday are considered to be in week 0.}{}
  \lineiii{\%x}{Locale's appropriate date representation.}{}
  \lineiii{\%X}{Locale's appropriate time representation.}{}
  \lineiii{\%y}{Year without century as a decimal number [00,99].}{}
  \lineiii{\%Y}{Year with century as a decimal number.}{}
  \lineiii{\%Z}{Time zone name (no characters if no time zone exists).}{}
  \lineiii{\%\%}{A literal \character{\%} character.}{}
\end{tableiii}

\noindent
Notes:

\begin{description}
  \item[(1)]
    The range really is \code{0} to \code{61}; this accounts for leap
    seconds and the (very rare) double leap seconds.
\end{description}

Here is an example, a format for dates compatible with that specified 
in the \rfc{2822} Internet email standard.
	\footnote{The use of \code{\%Z} is now
	deprecated, but the \code{\%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}.}

\begin{verbatim}
>>> from time import gmtime, strftime
>>> strftime("%a, %d %b %Y %H:%M:%S +0000", gmtime())
'Thu, 28 Jun 2001 14:17:15 +0000'
\end{verbatim}

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 \character{\%} of a
directive in the following order; this is also not portable.
The field width is normally 2 except for \code{\%j} where it is 3.
\end{funcdesc}

\begin{funcdesc}{strptime}{string\optional{, format}}
Parse a string representing a time according to a format.  The return 
value is a \class{struct_time} as returned by \function{gmtime()} or
\function{localtime()}.  The \var{format} parameter uses the same
directives as those used by \function{strftime()}; it defaults to
\code{"\%a \%b \%d \%H:\%M:\%S \%Y"} which matches the formatting
returned by \function{ctime()}.  The same platform caveats apply; see
the local \UNIX{} documentation for restrictions or additional
supported directives.  If \var{string} cannot be parsed according to
\var{format}, \exception{ValueError} is raised.  Values which are not
provided as part of the input string are filled in with default
values; the specific values are platform-dependent as the XPG standard
does not provide sufficient information to constrain the result.
\end{funcdesc}

\begin{datadesc}{struct_time}
The type of the time value sequence returned by \function{gmtime()},
\function{localtime()}, and \function{strptime()}.
\versionadded{2.2}
\end{datadesc}

\begin{funcdesc}{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.
\end{funcdesc}

\begin{datadesc}{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).
\end{datadesc}

\begin{datadesc}{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.
\end{datadesc}


\begin{seealso}
  \seemodule{locale}{Internationalization services.  The locale
                     settings can affect the return values for some of 
                     the functions in the \module{time} module.}
  \seemodule{calendar}{General calendar-related functions.  
                       \function{timegm()} is the inverse of
                       \function{gmtime()} from this module.}
\end{seealso}