void tzset (void);
extern char *tzname;
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 envi-
ronment 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).
If the TZ variable does not appear in the environment, the system time-
zone is used. The system timezone is configured by copying, or link-
ing, 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.
The value of TZ can be one of two formats. The first format is a
string of characters that directly represent the timezone to be used:
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 characters 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:
The dst string and offset specify the name and offset for the corre-
sponding daylight saving timezone. If the offset is omitted, it
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
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:
The second format specifies that the timezone information should be
read from a file:
If the file specification filespec is omitted, or its value cannot be
interpreted, then Coordinated Universal Time (UTC) is used. If file-
spec 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. If
the colon is omitted each of the above TZ formats will be tried.
Here's an example, once more for New Zealand:
TZ If this variable is set its value takes precedence over the sys-
tem configured timezone.
TZDIR If this variable is set its value takes precedence over the sys-
tem configured timezone database directory path.
The system timezone file.
The system timezone database directory.
When a TZ string includes a dst timezone without anything fol-
lowing 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 config-
POSIX.1-2001, POSIX.1-2008, 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, other-
wise the daylight saving time version.
date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
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