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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | ENVIRONMENT | FILES | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | BUGS | SEE ALSO | COLOPHON |
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tzset(3) Library Functions Manual tzset(3)
tzset, tzname, timezone, daylight - initialize time conversion
information
Standard C library (libc, -lc)
#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
tzname:
_POSIX_C_SOURCE
timezone, daylight:
_XOPEN_SOURCE
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_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, past, present, or future when daylight saving
time applies).
The tzset() function initializes these variables to unspecified
values if this timezone is a geographical timezone like
"America/New_York" (see below).
If the TZ variable does not appear in the environment, the system
timezone is used. The system timezone is configured by copying,
or linking, a file in the tzfile(5) format to /etc/localtime. A
timezone database of these files may be located in the system
timezone directory (see the FILES section below).
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, then Coordinated Universal Time (UTC) is
used.
A nonempty value of TZ can be one of two formats, either of which
can be preceded by a colon which is ignored. The first format is
a string of characters that directly represent the timezone to be
used:
std offset[dst[offset][,start[/time],end[/time]]]
There are no spaces in the specification. The std string
specifies an abbreviation for the timezone and must be three or
more alphabetic characters. When enclosed between the less-than
(<) and greater-than (>) signs, the character set is expanded to
include the plus (+) sign, the minus (-) sign, and digits. 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 00 and 59:
[+|-]hh[:mm[:ss]]
The dst string and offset specify the name and offset for the
corresponding daylight saving timezone. If the offset is omitted,
it defaults 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.
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.
n This specifies the zero-based Julian day with n between 0
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. They use the same
format as offset except that the hour can be in the range [-167,
167] to represent times before and after the named day. 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 September's last Sunday, at the
default time 02:00:00, to April's first Sunday at 03:00:00.
TZ="NZST-12:00:00NZDT-13:00:00,M9.5.0,M4.1.0/3"
The second —or "geographical"— format specifies that the timezone
information should be read from a file:
filespec
The filespec specifies a 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. If the specified file cannot be read or interpreted,
Coordinated Universal Time (UTC) is used; however, applications
should not depend on random filespec values standing for UTC, as
TZ formats may be extended in the future.
Here's an example, once more for New Zealand:
TZ="Pacific/Auckland"
TZ If this variable is set its value takes precedence over the
system configured timezone.
TZDIR If this variable is set its value takes precedence over the
system configured timezone database directory path.
/etc/localtime
The system timezone file.
/usr/share/zoneinfo/
The system timezone database directory.
/usr/share/zoneinfo/posixrules
When a TZ string includes a dst timezone without anything
following it, then this file is used for the start/end
rules. It is in the tzfile(5) format. By default, the
zoneinfo Makefile hard links it to the America/New_York
tzfile.
Above are the current standard file locations, but they are
configurable when glibc is compiled.
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────┬───────────────┬────────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────┼───────────────┼────────────────────┤
│ tzset() │ Thread safety │ MT-Safe env locale │
└───────────────────────────┴───────────────┴────────────────────┘
POSIX.1-2024.
tzset()
tzname POSIX.1-1988, SVr4, 4.3BSD.
timezone
daylight
POSIX.1-2001 (XSI), SVr4, 4.3BSD.
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.
Because the values of tzname, timezone, and daylight are often
unspecified, and accessing them can lead to undefined behavior in
multithreaded applications, code should instead obtain time zone
offset and abbreviations from the tm_gmtoff and tm_zone members of
the broken-down time structure tm(3type).
Since this function does not report errors, there's no way to know
if the value of TZ represents a valid time zone.
date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 tzset(3)
Pages that refer to this page: homectl(1), pmlogrewrite(1), gettimeofday(2), ctime(3), timegm(3), tm(3type), localtime(5), tzfile(5), environ(7), anacron(8), hwclock(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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