--- name: "tzset" section: "3" platform: "macos" package: "Libc" version: "1752.120.2" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/tzset" --- # tzset(3) initialize time conversion information ## NAME **tzset**, **tzsetwall** – initialize time conversion information ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *void*\ **tzset**(*void*); *void*\ **tzsetwall**(*void*); ## DESCRIPTION The **tzset**() function initializes time conversion information used by the library routine [localtime(3)](/localtime/3.md). The environment variable TZ specifies how this is done. If TZ does not appear in the environment, the best available approximation to local wall clock time, as specified by the [tzfile(5)](/tzfile/5.md)-format file #ifdef UNIFDEF\_MOVE\_LOCALTIME */var/db/timezone/localtime*, #else /\* !UNIFDEF\_MOVE\_LOCALTIME \*/ */etc/localtime*, #endif /\* UNIFDEF\_MOVE\_LOCALTIME \*/ is used. If TZ appears in the environment but its value is a null string, Coordinated Universal Time (UTC) is used (without leap second correction). If TZ appears in the environment and its value begins with a colon (‘:’), the rest of its value is used as a pathname of a [tzfile(5)](/tzfile/5.md)-format file from which to read the time conversion information. If the first character of the pathname is a slash (‘/’), it is used as an absolute pathname; otherwise, it is used as a pathname relative to the system time conversion information directory. If its value does not begin with a colon, it is first used as the pathname of a file (as described above) from which to read the time conversion information. If that file cannot be read, the value is then interpreted as a direct specification (the format is described below) of the time conversion information. If the TZ environment variable does not specify a [tzfile(5)](/tzfile/5.md)-format file and cannot be interpreted as a direct specification, UTC is used. The **tzsetwall**() function sets things up so that localtime returns the best available approximation of local wall clock time. ## SPECIFICATION FORMAT When TZ is used directly as a specification of the time conversion information, it must have the following syntax (spaces inserted for clarity): *std* *offset*\[ *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. 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* 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 no leading sign (‘-’) or (‘+’) is allowed. The default, if *time* is not given, is **02:00:00**. If no *rule* is present in the TZ specification, the rules specified by the [tzfile(5)](/tzfile/5.md)-format file *posixrules* in the system time conversion information directory 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. ## FILES \#ifdef UNIFDEF\_TZDIR\_SYMLINK #else /\* !UNIFDEF\_TZDIR\_SYMLINK \*/ #endif /\* UNIFDEF\_TZDIR\_SYMLINK \*/ - */etc/localtime* local time zone file - */usr/share/zoneinfo* time zone directory - */usr/share/zoneinfo/posixrules* rules for POSIX-style TZ's - */usr/share/zoneinfo/GMT* for UTC leap seconds If the file */usr/share/zoneinfo/GMT* does not exist, UTC leap seconds are loaded from */usr/share/zoneinfo/posixrules*. ## SEE ALSO [date(1)](/date/1.md), [gettimeofday(2)](/gettimeofday/2.md), [ctime(3)](/ctime/3.md), [getenv(3)](/getenv/3.md), [time(3)](/time/3.md), [tzfile(5)](/tzfile/5.md) ## HISTORY The **tzset**() and **tzsetwall**() functions first appeared in 4.4BSD.