— initialize time conversion
extern char *tzname;
extern long timezone;
extern long daylight;
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
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.
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.
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
TZ appears in the environment and its
value does not begin with a colon, it is first used as the pathname of a
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
TZ is set to the empty string,
Coordinated Universal Time (UTC) is used (without leap second
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
TZ is used directly as a
specification of the time conversion information, it must have the following
syntax (without whitespace between std and
- 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
If no rule is present in
TZ, the rules specified by the
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
For compatibility with System V Release 3.1, a semicolon
;’) may be used to separate the
rule from the rest of the specification.
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
- 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)
tzset() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”). The
tzsetwall() function is an extension to that