TZSET(3)

NAME
tzset - initialize time conversion information

LIBRARY
libc

SYNOPSIS
#include <time.h>
void
tzset(void);

DESCRIPTION
The tzset() function uses the value of the environment variable TZ to set time conversion information used by localtime(3). If TZ does not appear in the environment, the best available approximation to local wall clock time, as specified by the tzfile(5) format file /etc/localtime is used by localtime(3). If TZ appears in the environment but its value is a null string, Universal Time (UT) is used, with the abbreviation “UTC” and without leap second correction; please see ctime(3). If TZ appears in the environment and its value is not a null string:

-   if the value begins with a colon, it is used as a pathname of a file from which to read the time conversion information;

-   if the value does not begin with a colon, it is first used as the pathname of a file from which to read the time conversion information, and, if that file cannot be read, is used directly as a specification of the time conversion information.

When TZ is used as a pathname, if it begins with a slash, it is used as an absolute pathname; otherwise, it is used as a pathname relative to /usr/share/zoneinfo. The file must be in the format specified in tzfile(5).

When TZ is used directly as a specification of the time conversion information, it must have the following syntax (spaces inserted for clarity):

    stdoffset[
    dst[
    offset][
    ,rule]]

where:
std and dst
    Three or more bytes that are the designation for the standard (std) or summer (dst) time zone. Only std is required; if dst is missing, then summer time does not apply in this locale. Upper- and lowercase letters are explicitly allowed. Any characters except a leading colon (:), digits, comma (,), minus (-), plus (+), and ASCII NUL are allowed.
offset
    Indicates the value one must add to the local time to arrive at Coordinated Universal Time. The offset has the form:

        hh[
        :mm[
        :ss]]

    The minutes (mm) and seconds (ss) are optional. The hour (hh) is required and may be a single digit. The offset following std is required. If no offset follows dst, summer time is assumed to be one hour ahead of standard time. One or more digits may be used; the value is always interpreted as a decimal number. The hour must be between zero and 24, and the minutes (and seconds) — if present — between zero and 59. If preceded by a “-” the time zone shall be east of the Prime Meridian; otherwise it shall be west (which may be indicated by an optional preceding “+”).
rule
    Indicates when to change to and back from summer time. The rule has the form:

        date/
        time,
        date/
        time

    where the first date describes when the change from standard to summer time occurs and the second date describes when the change back happens. Each time field describes when, in current local time, the change to the other time is made. As an extension to POSIX, daylight saving is assumed to be in effect all year if it begins January 1 at 00:00 and ends December 31 at 24:00 plus the difference between daylight saving and standard time, leaving no room for standard time in the calendar. The format of date is one of the following:
    Jn  The Julian day n (1 n 365). Leap days are not counted; that is, in all years — including leap years — February 28 is day 59 and March 1 is day 60. It is impossible to explicitly refer to the occasional February 29.
    n   The zero-based Julian day (0 n 365). Leap days are counted, and it is possible to refer to February 29.
    Mm.n.d
        The d'th day (0 d 6) of week n of month m of the year (1 n 5, 1 m 12, where week 5 means “the last d day in month m” which may occur in either the fourth or the fifth week). Week 1 is the first week in which the d'th day occurs. Day zero is Sunday.
    The time has the same format as offset except that POSIX does not allow a leading sign “-” or “+” is allowed. As an extension to POSIX, the hours part of time can range from -167 through 167; this allows for unusual rules such as “the Saturday before the first Sunday of March”. The default, if time is not given, is 02:00:00.

Here are some examples of TZ values that directly specify the time zone rules; they use some of the extensions to POSIX.

EST5
    stands for US eastern time (EST), 5 hours behind UTC, without daylight saving.

FJT-12FJST,M10.3.1/146,M1.3.4/75
    stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours ahead of UTC, springing forward on October's third Monday at 146:00 (i.e., 02:00 on the first Sunday on or after October 21), and falling back on January's third Thursday at 75:00 (i.e., 03:00 on the first Sunday on or after January 18).

IST-2IDT,M3.4.4/26,M10.5.0
    stands for Israel Standard Time (IST) and Israel Daylight Time (IDT), 2 hours ahead of UTC, springing forward on March's fourth Tuesday at 26:00 (i.e., 02:00 on the first Friday on or after March 23), and falling back on October's last Sunday at 02:00.

WART4WARST,J1/0,J365/25
    stands for Western Argentina Summer Time (WARST), 3 hours behind UTC. There is a dummy fall-back transition on December 31 at 25:00 daylight saving time (i.e., 24:00 standard time, equivalent to January 1 at 00:00 standard time), and a simultaneous spring-forward transition on January 1 at 00:00 standard time, so daylight saving time is in effect all year and the initial WART is a placeholder.

WGT3WGST,M3.5.0/-2,M10.5.0/-1
    stands for Western Greenland time (WGT) and Western Greenland Summer Time (WGST), 3 hours behind UTC, where clocks follow the EU rules of springing forward on March's last Sunday at 01:00 UTC (-02:00 local time) and falling back on October's last Sunday at 01:00 UTC (-01:00 local time).

If no rule is present in TZ, the rules specified by the tzfile(5) format file posixrules in /usr/share/zoneinfo are used, with the standard and summer time offsets from UTC replaced by those specified by the offset values in TZ.

For compatibility with System V Release 3.1, a semicolon (;) may be used to separate the rule from the rest of the specification.

If the TZ environment variable does not specify a tzfile(5) format file and cannot be interpreted as a direct specification, UTC is used.

FILES
/etc/localtime
    local time zone file
/usr/share/zoneinfo
    time zone information directory
/usr/share/zoneinfo/posixrules
    used with POSIX-style TZ's
/usr/share/zoneinfo/GMT
    for UTC leap seconds

If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/share/zoneinfo/posixrules.

SEE ALSO
ctime(3), getenv(3), strftime(3), time(3), tzfile(5)

STANDARDS
The tzset() function conforms to p1003.1-88.