#include <time.h> void tzset (void); extern char *tzname; extern long timezone; extern int daylight;
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_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
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:
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:
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:
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:
The third format specifies that the timezone information should be read from a file:
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:
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.
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.