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TZSET(3) NetBSD Library Functions Manual TZSET(3)
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tzset, tzalloc, tzgetname, tzgetgmtoff, tzfree -- initialize time conver-
Standard C Library (libc, -lc)
tzalloc(const char *zone);
tzfree(timezone_t restrict tz);
const char *
tzgetname(timezone_t restrict tz, int isdst);
tzgetgmtoff(timezone_t restrict tz, int isdst);
The tzalloc() function takes as an argument a timezone name and returns a
timezone_t object suitable to be used in the ctime_rz(), localtime_rz(),
and mktime_z() functions.
If zone is not a valid timezone description, or if the object cannot be
allocated, tzalloc() returns a NULL pointer and sets errno.
A NULL pointer may be passed to tzalloc() instead of a timezone name, to
refer to the current system timezone. An empty timezone string indicates
Coordinated Universal Time (UTC).
Note that instead of setting the environment variable TZ, and globally
changing the behavior of the calling program, one can use multiple time-
zones at the same time by using separate timezone_t objects allocated by
tzalloc() and calling the ``z'' variants of the functions. The tzfree()
function deallocates tz, which was previously allocated by tzalloc().
This invalidates any tm_zone pointers that tz was used to set. The func-
tion tzgetname() returns the name for the given tz. If isdst is 0, the
call is equivalent to tzname. If isdst is set to 1 the call is equiv-
alent to tzname. The return values for both tzgetname() and
tzgmtoff() correspond to the latest time for which data is available,
even if that refers to a future time. Finally, the tzgetgmtoff() func-
tion acts like tzgetname() only it returns the offset in seconds from GMT
for the timezone. If there is no match, then -1 is returned and errno is
set to ESRCH. The tzset() function acts like tzalloc(getenv("TZ")),
except it saves any resulting timezone object into internal storage that
is accessed by localtime(), localtime_r(), and mktime(). The anonymous
shared timezone object is freed by the next call to tzset(). If the
implied call to tzalloc() fails, tzset() falls back on Universal Time
(UT). If TZ is NULL, 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
the empty string, UT is used, with the abbreviation ``UTC'' and without
leap second correction; please see ctime(3). If TZ is nonnull and
- 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 informa-
tion, and, if that file cannot be read, is used directly as a speci-
fication 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 infor-
mation, it must have the following syntax (spaces inserted for clarity):
std and dst Three or more bytes that are the designation for the stan-
dard (std) or the alternative (dst such as daylight saving
time) timezone. 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 NUL bytes are allowed. Alterna-
tively, a designation can be surrounded by angle brackets <
and >; in this case, the designation can contain any charac-
ters other than > and NUL.
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 fol-
lowing 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 timezone
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 daylight saving
time. The 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. 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 day-
light saving and standard time, leaving no room for standard
time in the calendar. The format of date is one of the fol-
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 POSIX
does not allow a leading sign ``-'' or ``+'' is allowed. 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 02:00:00.
Here are some examples of TZ values that directly specify the timezone
rules; they use some of the extensions to POSIX.
EST5 stands for US Eastern Standard Time (EST), 5 hours behind UT,
without daylight saving.
stands for Fiji time, 12 hours ahead of UT, springing forward on
November's first Sunday at 02:00, and falling back on January's
second Monday at 147:00 (i.e., 03:00 on the first Sunday on or
after January 14). The abbreviations for standard and daylight
saving time are "+12" and "+13".
stands for Israel Standard Time (IST) and Israel Daylight Time
(IDT), 2 hours ahead of UT, springing forward on March's fourth
Thursday 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 permanent daylight saving time, 3 hours behind UT with
abbreviation "-03". 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 daylight saving time is in effect all year and
the initial <-04> is a placeholder.
stands for time in western Greenland, 3 hours behind UT, where
clocks follow the EU rules of springing forward on March's last
Sunday at 01:00 UT (-02:00 local time, i.e., 22:00 the previous
day) and falling back on October's last Sunday at 01:00 UT
(-01:00 local time, i.e., 23:00 the previous day). The abbrevia-
tions for standard and daylight saving time are "-03" and "-02".
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
daylight saving time offsets from UT 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 timezone file
/usr/share/zoneinfo local timezone information directory
/usr/share/zoneinfo/posixrules default DST rules (obsolete, and can
cause bugs if present) used with POSIX-
/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'').
Neither the tzgetname() nor tzgmtoff() functions have the ability to
specify the point in time for which the requested data should be
NetBSD 10.99 September 16, 2023 NetBSD 10.99