TZSET(3) | Library Functions Manual | TZSET(3)

# NAME

`tzset`, `tzsetwall` — initialize time conversion information

# SYNOPSIS

`#include <time.h>`

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

void
`tzset`(void);

void
`tzsetwall`(void);

# DESCRIPTION

The `tzset`() 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:

tzname[2]
  the designations for standard and daylight saving time; see the description of std and dst below
timezone
  the number of seconds west of the Prime Meridian
daylight
  0 if the time zone has never observed daylight saving time, otherwise non-zero

Most programs do not need to call `tzset`() 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:

- the pathname of a tzfile(5) format file from which to read the time conversion information, optionally prefixed with a colon (‘`:`’), such as “:America/Denver” or “Europe/Berlin”
- a string that directly specifies the time conversion information (see below) which may not begin with a colon (‘`:`’)

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, Universal Time (UT) is used, with the abbreviation “UTC” and without leap second correction; please see ctime(3) for more about UT, UTC, and leap seconds.

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]

Where:

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.
offset
  Indicates the value one must add to the local time to arrive at Coordinated Universal Time. 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, 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 “+”).

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

  date/time,date/time

  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. 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:

  `J` n
    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.
  `M` m.n.d
    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 POSIX does not allow a leading sign (“-” or “+”). 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 Standard Time (EST), 5 hours behind UT, 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 UT, 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 UT, 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 UT. 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 UT, where clocks follow the EU rules of springing forward on March's last Sunday at 01:00 UT (-02:00 local time) and falling back on October's last Sunday at 01:00 UT (-01:00 local time).

  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 UT 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, UT is used.

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

# FILES

/usr/share/zoneinfo
  time zone information directory
/etc/localtime
  local time zone file
/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 IEEE Std 1003.1-2008 (“POSIX.1”). The `tzsetwall`() function is an extension to that specification.

June 23, 2025 | Debian