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TZSET(3) NetBSD Library Functions Manual TZSET(3)
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tzset, tzalloc, tzgetname, tzfree -- initialize time conversion informa-
Standard C Library (libc, -lc)
tzalloc(const char *zone);
tzfree(timezone_t restrict tz);
const char *
tzgetname(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 tz is not a valid time zone 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. Finally,
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 equivalent
to tzname. The tzset() function acts like tzalloc(getenv("TZ")),
except it saves any resulting time zone object into internal storage that
is accessed by localtime(), localtime_r(), and mktime(). The anonymous
shared time zone object is freed by the next call to tzset(). If the
implied call to tzalloc() fails, tzset() falls back on UTC.
localtime(3). 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, Universal Time (UT) is used, with the abbreviation
``UTC'' and without leap second correction; please see ctime(3). If TZ
is nonnull and nonempty:
- 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 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 NUL bytes 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 fol-
lowing 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.
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
saving and standard time, leaving no room for standard time
in the calendar. The format of date is one of the follow-
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 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 November's first
Sunday at 02:00, and falling back on January's third Thurs-
day at 75:00 (i.e., 03:00 on the first Sunday on or after
stands for Israel Standard Time (IST) and Israel Daylight
Time (IDT), 2 hours ahead of UTC, 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 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 Jan-
uary 1 at 00:00 standard time, so daylight saving time is
in effect all year and the initial WART is a placeholder.
stands for Western Greenland Time (WGT) and Western Green-
land Summer 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 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.
/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'').
NetBSD 7.1.2 October 23, 2014 NetBSD 7.1.2