tzset(3) - Linux manual page (original) (raw)
tzset(3) Library Functions Manual tzset(3)
NAME top
tzset, tzname, timezone, daylight - initialize time conversion
informationLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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_SOURCEDESCRIPTION top
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)](../man5/tzfile.5.html) 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:
J_n_ 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.
M_m_._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)](../man5/tzfile.5.html)-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"ENVIRONMENT top
**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.FILES top
_/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)](../man5/tzfile.5.html) format. By default, the
zoneinfo Makefile hard links it to the _America/NewYork_
tzfile.
Above are the current standard file locations, but they are
configurable when glibc is compiled.ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌───────────────────────────┬───────────────┬────────────────────┐
│ **Interface** │ **Attribute** │ **Value** │
├───────────────────────────┼───────────────┼────────────────────┤
│ **tzset**() │ Thread safety │ MT-Safe env locale │
└───────────────────────────┴───────────────┴────────────────────┘STANDARDS top
POSIX.1-2024.HISTORY top
**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.CAVEATS top
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 _tmgmtoff_ and _tmzone_ members of
the broken-down time structure [tm(3type)](../man3/tm.3type.html).BUGS top
Since this function does not report errors, there's no way to know
if the value of TZ represents a valid time zone.SEE ALSO top
[date(1)](../man1/date.1.html), [gettimeofday(2)](../man2/gettimeofday.2.html), [time(2)](../man2/time.2.html), [ctime(3)](../man3/ctime.3.html), [getenv(3)](../man3/getenv.3.html), [tzfile(5)](../man5/tzfile.5.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-08-30 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)