tzset - initialize time conversion information
Standard C Library (libc, -lc)
The tzset() function uses the value of the environment variable TZ to set
time conversion information used by localtime(3). If TZ does not appear
in the environment, the best available approximation to local wall clock
time, as specified by the tzfile(5) format file /etc/localtime is used by
localtime(3). 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 is not a
- if the value begins with a colon, it is used as a pathname of a file
from which to read the time conversion information;
- if the value does not begin with a colon, it is first used as the
pathname of a 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.
When TZ is used as a pathname, if it begins with a slash, it is used as
an absolute pathname; otherwise, it is used as a pathname relative to
/usr/share/zoneinfo. The file must be in the format specified in
When TZ is used directly as a specification of the time conversion information,
it must have the following syntax (spaces inserted for clarity):
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:
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:
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:
Jn 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.
Mm.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 TZ, the rules specified by the tzfile(5) format
file posixrules in /usr/share/zoneinfo 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.
If the TZ environment variable does not specify a tzfile(5) format file
and cannot be interpreted as a direct specification, UTC is used.
/etc/localtime local time zone file
/usr/share/zoneinfo time zone information directory
/usr/share/zoneinfo/posixrules used with POSIX-style TZ's
/usr/share/zoneinfo/GMT for UTC leap seconds
If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
ctime(3), getenv(3), strftime(3), time(3), tzfile(5)
The tzset() function conforms to IEEE Std 1003.1-1988 (``POSIX.1'').
BSD April 1, 2001 BSD
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