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TZSET(3) Library Functions Manual TZSET(3)

tzset, tzsetwallinitialize time conversion information

#include <time.h>

extern char *tzname[2];
extern long timezone;
extern long daylight;



The () function uses the value of the environment variable TZ to set the time conversion information used by localtime(3). It also sets the following external variables:

the designations for standard and daylight saving time; see the description of std and dst below
the number of seconds west of UTC
0 if the time zone has never observed daylight saving time, otherwise non-zero

Most programs do not need to call () directly; it will be called automatically as needed by the functions described in localtime(3). Privileged processes that use chroot(2) may wish to call tzset() to initialize the time conversion information before changing to a restricted root directory that does not include time conversion data files.

If TZ does not appear in the environment, or if the calling process has changed its user or group ID, the system time zone file, /etc/localtime, is used.

If TZ appears in the environment it may be one of two formats:

If TZ appears in the environment and its value does not begin with a colon, it is first used as the pathname of a tzfile(5) format 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. A value beginning with a colon (‘:’) is always treated as a pathname.

If TZ is set to the empty string, Coordinated Universal Time (UTC) is used (without leap second correction).

When TZ is used as a pathname, it must either be a path relative to the system time conversion information directory, /usr/share/zoneinfo, or an absolute path that begins with /usr/share/zoneinfo/. Other absolute paths, or paths that contain ‘../’, will be ignored and the system local time zone file, /etc/localtime, will be used instead. 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 (without whitespace between std and offset):

std offset[dst[offset]][,rule]


std and dst
Three or more bytes that are the designation for the standard (std) or the daylight saving (dst) time zone. Only std is required; if dst is missing, then daylight saving 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.
Indicates the value one must add to the local time to arrive at Coordinated Universal Time. offset has the form:


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, daylight saving 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 “+”).

Indicates when to change to and back from daylight saving time. rule has the form:


where the first date describes when the change from standard to daylight saving 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.

The format of date is one of the following:

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.
The zero-based Julian day (0 <= n <= 365). Leap days are counted, and it is possible to refer to February 29.
Day d (1 <= d <= 6) of week n (1 <= n <= 5) of month m (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 dth day occurs. Day zero is Sunday.

The time has the same format as offset except that no leading sign (“-” or “+”) is allowed. The default, if time is not given, is 02:00:00.

If no rule is present in TZ, the rules specified by the tzfile(5) format file posixrules in the system time conversion information directory are used, with the standard and daylight saving 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.

() behaves identically to tzset() but it only uses the /etc/localtime file (that is, it ignores the TZ environment variable).

time zone information directory
local time zone file
used with POSIX-style TZs
for UTC leap seconds

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

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

The tzset() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”). The tzsetwall() function is an extension to that specification.

October 4, 2022 OpenBSD-7.5