#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 since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _SVID_SOURCE
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.
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 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:
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 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 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. If the colon is omitted each of the above TZ formats will be tried.
Here's an example, once more for New Zealand:
TZ=":Pacific/Auckland"
Above are the current standard file locations, but they are configurable when glibc is compiled.
| Interface | Attribute | Value |
| tzset() | Thread safety | MT-Safe env locale |
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.