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TZSET(3) Library Functions Manual TZSET(3) NAME daylight, timezone, tzname, tzset -- initialize time conversion infor- mation LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <time.h> extern int daylight; extern long timezone; extern char *tzname[2]; void tzset(void); DESCRIPTION The tzset() function initializes time conversion information used by the library routine localtime(3). The environment variable TZ speci- fies how this is done. If TZ does not appear in the environment, the best available approxima- tion to local wall clock time, as specified by the tzfile(5)-format file /etc/localtime is used. If TZ appears in the environment but its value is a null string, Coor- dinated 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)-format file from which to read the time conversion informa- tion. If the first character of the pathname is a slash (`/') it is used as an absolute pathname; otherwise, it is used as a pathname rela- tive to the system time conversion information directory. If its value does not begin with a colon, it is first used as the path- name of a file (as described above) from which to read the time conver- sion information. If that file cannot be read, the value is then in- terpreted 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)-format file and cannot be interpreted as a direct specification, UTC is used. After the first call to tzset, the timezone variable is set to the dif- ference, in seconds, between Coordinated Universal Time (UTC) and the local time. The daylight variable is set to 0 if the local timezone is observing standard time. It is set to 1 if the local timezone ever ob- serves an alternate time, such as summer time. The first element of the tzname array is the timezone name of standard time, while the sec- ond element is the name of the altnerative time zone. SPECIFICATION FORMAT When TZ is used directly as a specification of the time conversion in- formation, it must have the following syntax (spaces inserted for clar- ity): 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 ex- cept a leading colon (`:'), digits, comma (`,'), mi- nus (`-'), 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 impos- sible to explicitly refer to the occasional February 29. n The zero-based Julian day (0 <= n <= 365 ) . Leap days are counted, and it is possi- ble 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 ex- cept 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)-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 /etc/localtime local time zone file /usr/share/zoneinfo time zone directory /usr/share/zoneinfo/posixrules rules for POSIX-style TZs /usr/share/zoneinfo/Etc/GMT for UTC leap seconds If the file /usr/share/zoneinfo/UTC does not exist, UTC leap seconds are loaded from /usr/share/zoneinfo/posixrules. SEE ALSO date(1), gettimeofday(2), ctime(3), getenv(3), time(3), tzfile(5) HISTORY The tzset() function first appeared in 4.4BSD. FreeBSD 15.0 September 30, 2025 TZSET(3)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | SPECIFICATION FORMAT | FILES | SEE ALSO | HISTORY
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