TZSET(3) BSD Library Functions Manual TZSET(3)NAME
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, Coordi‐
nated 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 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 pre‐
ceded 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.
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 no leading sign (‘-’) or (‘+’) is
allowed. The default, if time is not given,
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.
/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 ALSOdate(1), gettimeofday(2), ctime(3), getenv(3), time(3), tzfile(5)HISTORY
The tzset() and tzsetwall() functions first appeared in 4.4BSD.
BSD November 17, 1993 BSD