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TZSET(3) Linux Programmer's Manual TZSET(3)
tzset, tzname, timezone, daylight - initialize time conversion information
#include <time.h>
void tzset (void);
extern char *tzname[2];
extern long timezone;
extern int daylight;
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
tzset(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
tzname: _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
timezone: _SVID_SOURCE || _XOPEN_SOURCE
daylight: _SVID_SOURCE || _XOPEN_SOURCE
The tzset() function initializes the tzname variable from the 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 timezone (seconds West of UTC) and daylight (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).
If the TZ variable does not appear in the environment, the tzname variable is
initialized with the best approximation of local wall clock time, as specified
by the tzfile(5)-format file localtime found in the system timezone directory
(see below). (One also often sees /etc/localtime used here, a symlink to the
right file in the system timezone directory.)
If the 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.
The value of TZ can be one of three formats. The first format is used when
there is no daylight saving time in the local timezone:
std offset
The std string specifies the name of the timezone and must be three or more
alphabetic characters. The offset string immediately follows std and
specifies the time value to be added to the local time to get Coordinated
Universal Time (UTC). The offset 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.
The second format is used when there is daylight saving time:
std offset dst [offset],start[/time],end[/time]
There are no spaces in the specification. The initial std and offset specify
the standard timezone, as described above. The dst string and offset 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.
The start field specifies when daylight saving time goes into effect and the
end field specifies when the change is made back to standard time. These
fields may have the following formats:
Jn This specifies the Julian day with n between 1 and 365. February 29 is
never counted even in leap years.
n This specifies the Julian day with n between 1 and 365. February 29 is
counted in leap years.
Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w <= 5) of month m
(1 <= m <= 12). Week 1 is the first week in which day d occurs and
week 5 is the last week in which day d occurs. Day 0 is a Sunday.
The time 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:
TZ="NZST-12.00:00NZDT-13:00:00,M10.1.0,M3.3.0"
The third format specifies that the timezone information should be read from a
file:
:[filespec]
If the file specification filespec is omitted, the timezone information is
read from the file localtime in the system timezone directory, which nowadays
usually is /usr/share/zoneinfo. This file is in tzfile(5) format. If
filespec is given, it specifies another tzfile(5)-format file to read the
timezone information from. If filespec does not begin with a '/', the file
specification is relative to the system timezone directory.
Here's an example, once more for New Zealand:
TZ=":Pacific/Auckland"
The system timezone directory used depends on the (g)libc version. Libc4 and
libc5 use /usr/lib/zoneinfo, and, since libc-5.4.6, when this doesn't work,
will try /usr/share/zoneinfo. Glibc2 will use the environment variable TZDIR,
when that exists. Its default depends on how it was installed, but normally
is /usr/share/zoneinfo.
This timezone directory contains the files
localtime local timezone file
posixrules rules for POSIX-style TZ's
Often /etc/localtime is a symlink to the file localtime or to the correct
timezone file in the system timezone directory.
SVr4, POSIX.1-2001, 4.3BSD.
Note that the variable daylight does not indicate that daylight saving time
applies right now. It used to give the number of some algorithm (see the
variable tz_dsttime in gettimeofday(2)). It has been obsolete for many years
but is required by SUSv2.
4.3BSD had a function 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.
date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
2010-02-25 TZSET(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface