.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" 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 .\" .TH TZSET 3 2014-08-19 "" "Linux Programmer's Manual" .SH NAME tzset, tzname, timezone, daylight \- initialize time conversion information .SH SYNOPSIS .nf .B #include .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 Under glibc, the system timezone directory is determined using the .BR TZDIR the environment variable. If .BR TZDIR the default depends on the system setup, but is normally .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 symbolic link 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) .SH COLOPHON This page is part of release 3.74 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%http://www.kernel.org/doc/man\-pages/.