man3/tzset.3

.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sun Jul 25 11:01:58 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified 2001-11-13, aeb
.\" Modified 2004-12-01 mtk and Martin Schulze <joey@infodrom.org>
.\"
.TH TZSET 3  2012-03-25 "" "Linux Programmer's Manual"
.SH NAME
tzset, tzname, timezone, daylight \- initialize time conversion information
.SH SYNOPSIS
.nf
.B #include <time.h>
.sp
.B void tzset (void);
.sp
.BI "extern char *" tzname [2];
.BI "extern long " timezone ;
.BI "extern int " daylight ;
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR tzset ():
_POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _POSIX_SOURCE
.br
.IR tzname :
_POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _POSIX_SOURCE
.br
.IR timezone :
_SVID_SOURCE || _XOPEN_SOURCE
.br
.IR daylight :
_SVID_SOURCE || _XOPEN_SOURCE
.SH DESCRIPTION
The
.BR tzset ()
function initializes the \fItzname\fP variable from the
.B TZ
environment variable.
This function is automatically called by the
other time conversion functions that depend on the timezone.
In a System-V-like environment, it will also set the variables \fItimezone\fP
(seconds West of UTC) and \fIdaylight\fP (to 0 if this timezone does not
have any daylight saving time rules, or to nonzero if there is a time during
the year when daylight saving time applies).
.PP
If the
.B TZ
variable does not appear in the environment, the \fItzname\fP
variable is initialized with the best approximation of local wall clock
time, as specified by the
.BR tzfile (5)-format
file \fIlocaltime\fP
found in the system timezone directory (see below).
(One also often sees
.I /etc/localtime
used here, a symlink to the right file in the system timezone directory.)
.PP
If the
.B TZ
variable does appear in the environment but its value is empty
or its value cannot be interpreted using any of the formats specified
below, Coordinated Universal Time (UTC) is used.
.PP
The value of
.B TZ
can be one of three formats.
The first format is used
when there is no daylight saving time in the local timezone:
.sp
.RS
.I std offset
.RE
.sp
The \fIstd\fP string specifies the name of the timezone and must be
three or more alphabetic characters.
The \fIoffset\fP string immediately
follows \fIstd\fP and specifies the time value to be added to the local
time to get Coordinated Universal Time (UTC).
The \fIoffset\fP is positive
if the local timezone is west of the Prime Meridian and negative if it is
east.
The hour must be between 0 and 24, and the minutes and seconds
0 and 59.
.PP
The second format is used when there is daylight saving time:
.sp
.RS
.I std offset dst [offset],start[/time],end[/time]
.RE
.sp
There are no spaces in the specification.
The initial \fIstd\fP and
\fIoffset\fP specify the standard timezone, as described above.
The \fIdst\fP string and \fIoffset\fP specify the name and offset for the
corresponding daylight saving timezone.
If the offset is omitted,
it default to one hour ahead of standard time.
.PP
The \fIstart\fP field specifies when daylight saving time goes into
effect and the \fIend\fP field specifies when the change is made back to
standard time.
These fields may have the following formats:
.TP
J\fIn\fP
This specifies the Julian day with \fIn\fP between 1 and 365.
Leap days are not counted.
In this format, February 29 can't be represented;
February 28 is day 59, and March 1 is always day 60.
.TP
.I n
This specifies the zero-based Julian day with \fIn\fP between 0 and 365.
February 29 is counted in leap years.
.TP
M\fIm\fP.\fIw\fP.\fId\fP
This specifies day \fId\fP (0 <= \fId\fP <= 6) of week \fIw\fP
(1 <= \fIw\fP <= 5) of month \fIm\fP (1 <= \fIm\fP <= 12).
Week 1 is
the first week in which day \fId\fP occurs and week 5 is the last week
in which day \fId\fP occurs.
Day 0 is a Sunday.
.PP
The \fItime\fP fields specify when, in the local time currently in effect,
the change to the other time occurs.
If omitted, the default is 02:00:00.

Here is an example for New Zealand,
where the standard time (NZST) is 12 hours ahead of UTC,
and daylight saving time (NZDT), 13 hours ahead of UTC,
runs from the first Sunday in October to the third Sunday in March,
and the changeovers happen at the default time of 02:00:00:
.nf

    TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
.fi
.PP
The third format specifies that the timezone information should be read
from a file:
.sp
.RS
:[filespec]
.RE
.sp
If the file specification \fIfilespec\fP is omitted, the timezone
information is read from the file
.I localtime
in the system timezone directory, which nowadays usually is
.IR /usr/share/zoneinfo .
This file is in
.BR tzfile (5)
format.
If \fIfilespec\fP is given, it specifies another
.BR tzfile (5)-format
file to read the timezone information from.
If \fIfilespec\fP does not begin with a \(aq/\(aq, the file specification is
relative to the system timezone directory.
.PP
Here's an example, once more for New Zealand:
.nf

    TZ=":Pacific/Auckland"
.fi
.SH FILES
The system timezone directory used depends on the (g)libc version.
Libc4 and libc5 use
.IR /usr/lib/zoneinfo ,
and, since libc-5.4.6,
when this doesn't work, will try
.IR /usr/share/zoneinfo .
Glibc2 will use the environment variable
.BR TZDIR ,
when that exists.
Its default depends on how it was installed, but normally is
.IR /usr/share/zoneinfo .
.LP
This timezone directory contains the files
.nf
localtime      local timezone file
posixrules     rules for POSIX-style TZ's
.fi
.LP
Often
.I /etc/localtime
is a symlink to the file
.I localtime
or to the correct timezone file in the system timezone directory.
.SH "CONFORMING TO"
SVr4, POSIX.1-2001, 4.3BSD.
.SH NOTES
Note that the variable \fIdaylight\fP does not indicate that daylight
saving time applies right now.
It used to give the number of some
algorithm (see the variable \fItz_dsttime\fP in
.BR gettimeofday (2)).
It has been obsolete for many years but is required by SUSv2.
.LP
4.3BSD had a function
.BI "char *timezone(" zone ", " dst )
that returned the
name of the timezone corresponding to its first argument (minutes
West of UTC).
If the second argument was 0, the standard name was used,
otherwise the daylight saving time version.
.SH "SEE ALSO"
.BR date (1),
.BR gettimeofday (2),
.BR time (2),
.BR ctime (3),
.BR getenv (3),
.BR tzfile (5)