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TZSET(3) DragonFly Library Functions Manual TZSET(3)
tzset, tzsetwall -- initialize time conversion information
Standard C Library (libc, -lc)
The tzset() function initializes time conversion information used by the
library routine localtime(3). The environment variable TZ specifies 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 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.
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 information. If the first charac-
ter 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 conver-
sion 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 inter-
preted 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.
The tzsetwall() function sets things up so that localtime(3) returns the
best available approximation of local wall clock time.
When TZ is used directly as a specification of the time conversion infor-
mation, it must have the following syntax (spaces inserted for clarity):
std offset [dst [offset] [, rule]]
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 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 fol-
lows 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:
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. 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 sav-
ing 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
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 Sun-
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
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 UTC,
without daylight saving.
stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours
ahead of UTC, 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).
stands for Israel Standard Time (IST) and Israel Daylight Time
(IDT), 2 hours ahead of UTC, 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.
stands for Western Argentina Summer Time (WARST), 3 hours behind
UTC. 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 day-
light saving time is in effect all year and the initial WART is a
stands for Western Greenland Time (WGT) and Western Greenland Sum-
mer Time (WGST), 3 hours behind UTC, where clocks follow the EU
rules of springing forward on March's last Sunday at 01:00 UTC
(-02:00 local time) and falling back on October's last Sunday at
01:00 UTC (-01:00 local time).
If no rule is present in the TZ specification, the rules specified by the
tzfile(5)-format file posixrules in the system time conversion informa-
tion 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.
/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.
date(1), gettimeofday(2), ctime(3), getenv(3), strftime(3), time(3),
The tzset() and tzsetwall() functions first appeared in 4.4BSD.
DragonFly 3.7 December 1, 2013 DragonFly 3.7