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TZSET(3)                   Library Functions Manual                   TZSET(3)

     tzset, tzalloc, tzgetname, tzgetgmtoff, tzfree - initialize time
     conversion information

     Standard C Library (libc, -lc)

     #include <time.h>

     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
     timezones 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
     function tzgetname() returns the name for the given tz.  If isdst is 0,
     the call is equivalent to tzname[0].  If isdst is set to 1 the call is
     equivalent to tzname[1].  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()
     function 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 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
         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


     std and dst  Three or more bytes that are the designation for the
                  standard (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.
                  Alternatively, a designation can be surrounded by angle
                  brackets < and >; in this case, the designation can contain
                  any characters 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
                  following 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
                  daylight saving and standard time, leaving no room for
                  standard time in the calendar.  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 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
             abbreviations for standard and daylight saving time are "-03" and

     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-
                                     style TZ
     /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