tzset man page on Alpinelinux

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NEWTZSET(3)							   NEWTZSET(3)

NAME
       tzset - initialize time conversion information

SYNOPSIS
       void tzset()

       cc ... -ltz

DESCRIPTION
       Tzset uses the value of the environment variable TZ to set time conver‐
       sion information used by localtime.  If TZ does not appear in the envi‐
       ronment,	 the best available approximation to local wall clock time, as
       specified by the tzfile(5)-format file localtime	 in  the  system  time
       conversion  information directory, is used by localtime.	 If TZ appears
       in the environment but its value is a null string, Universal Time  (UT)
       is  used,  with	the abbreviation "UTC" and without leap second correc‐
       tion; please see newctime(3) for more about UT, UTC, and leap  seconds.
       If TZ appears in the environment and its value is not a null string:

	      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 a
       system  time conversion information directory.  The file must be in the
       format specified in tzfile(5).

       When TZ is used directly as a  specification  of	 the  time  conversion
       information,  it	 must  have  the following syntax (spaces inserted for
       clarity):

	      stdoffset[dst[offset][,rule]]

       Where:

	      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:

				    hh[:mm[:ss]]

			     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	inter‐
			     preted  as	 a  decimal  number.  The hour must be
			     between zero and 24, and the  minutes  (and  sec‐
			     onds)  —  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:

				    date/time,date/time

			     where the first date describes  when  the	change
			     from  standard to summer time occurs and the sec‐
			     ond 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 29.

			     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	 ``+'').   As an extension to POSIX, the hours
			     part of time can range  from  −167	 through  167;
			     this allows for unusual rules such as "the Satur‐
			     day 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.

       FJT−12FJST,M10.3.1/146,M1.3.4/75
	      stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours
	      ahead of UTC, springing forward on  October's  third  Monday  at
	      146:00 (i.e., 02:00 on the first Sunday on or after October 21),
	      and falling back on January's third  Thursday  at	 75:00	(i.e.,
	      03:00 on the first Sunday on or after January 18).

       IST−2IDT,M3.4.4/26,M10.5.0
	      stands  for  Israel Standard Time (IST) and Israel Daylight Time
	      (IDT), 2 hours ahead of UTC, springing forward on March's fourth
	      Tuesday  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.

       WART4WARST,J1/0,J365/25
	      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,  equiva‐
	      lent  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
	      WART is a placeholder.

       WGT3WGST,M3.5.0/−2,M10.5.0/−1
	      stands for Western Greenland Time (WGT)  and  Western  Greenland
	      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 Sun‐
	      day at 01:00 UTC (−01:00 local time).

       If no rule is present in TZ, the rules specified by the	tzfile(5)-for‐
       mat 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.

       If the TZ environment variable does not specify a tzfile(5)-format  and
       cannot be interpreted as a direct specification, UTC is used.

FILES
       /usr/local/etc/zoneinfo		   time zone information directory
       /usr/local/etc/zoneinfo/localtime   local time zone file
       /usr/local/etc/zoneinfo/posixrules  used with POSIX-style TZ's
       /usr/local/etc/zoneinfo/GMT	   for UTC leap seconds

       If  /usr/local/etc/zoneinfo/GMT	is absent, UTC leap seconds are loaded
       from /usr/local/etc/zoneinfo/posixrules.

SEE ALSO
       getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)

								   NEWTZSET(3)
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