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TZSET(3)		   Linux Programmer's Manual		      TZSET(3)



NAME
       tzset, tzname, timezone, daylight - initialize time conversion informa-
       tion

SYNOPSIS
       #include <time.h>

       void tzset (void);

       extern char *tzname[2];
       extern long timezone;
       extern int daylight;

DESCRIPTION
       The tzset() function initializes the tzname variable from the TZ	 envi-
       ronment	variable.   This function is automatically called by the other
       time conversion functions that depend on the time zone.	In a SysV-like
       environment  it	will  also set the variables timezone (seconds West of
       GMT) and daylight (0 if this time zone does not have any daylight  sav-
       ings  time rules, non-zero if there is a time during the year when day-
       light savings time applies).

       If the TZ variable does not appear in the environment, the tzname vari-
       able  is	 initialized  with  the best approximation of local wall clock
       time, as specified by the tzfile(5)-format file localtime found in  the
       system	timezone   directory   (see  below).   (One  also  often  sees
       /etc/localtime used here, a symlink to the right	 file  in  the	system
       timezone directory.)

       If  the	TZ  variable  does  appear in the environment but its value is
       empty or its value cannot be interpreted using any of the formats spec-
       ified below, Coordinated Universal Time (UTC) is used.

       The  value of TZ can be one of three formats.  The first format is used
       when there is no daylight saving time in the local time zone:

	      std offset

       The std string specifies the name of the time zone and must be three or
       more  alphabetic characters.  The offset string immediately follows std
       and specifies the time value to be added to the local time to get Coor-
       dinated Universal Time (UTC).  The offset is positive if the local time
       zone is west of the Prime Meridian and negative if  it  is  east.   The
       hour must be between 0 and 24, and the minutes and seconds 0 and 59.

       The second format is used when there is daylight saving time:

	      std offset dst [offset],start[/time],end[/time]

       There  are  no spaces in the specification.  The initial std and offset
       specify the standard time zone, as described above.  The dst string and
       offset  specify the name and offset for the corresponding daylight sav-
       ings time zone.	If the offset is omitted, it  defaults	 to  one  hour
       ahead of standard time.

       The  start  field specifies when daylight savings time goes into effect
       and the end field specifies when the change is made  back  to  standard
       time.  These fields may have the following formats:

       Jn     This  specifies the Julian day with n between 1 and 365.	Febru-
	      ary 29 is never counted even in leap years.

       n      This specifies the Julian day with n between 0 and 365.	Febru-
	      ary 29 is counted in leap years.

       Mm.w.d This  specifies  day  d (0 <= d <= 6) of week w (1 <= w <= 5) of
	      month m (1 <= m <= 12).  Week 1 is the first week in which day d
	      occurs and week 5 is the last week in which day d occurs.	 Day 0
	      is a Sunday.

       The time fields specify when, in the local time	currently  in  effect,
       the  change  to	the  other  time  occurs.   If omitted, the default is
       02:00:00.

       The third format specifies that the time	 zone  information  should  be
       read from a file:

	      :[filespec]

       If  the	file specification filespec is omitted, the time zone informa-
       tion is read from the file localtime in the system timezone  directory,
       which  nowadays	usually	 is  /usr/share/zoneinfo.   This  file	is  in
       tzfile(5)  format.   If	filespec  is  given,  it   specifies   another
       tzfile(5)-format file to read the time zone information from.  If file-
       spec does not begin with a '/', the file specification is  relative  to
       the system timezone directory.

FILES
       The  system  time  zone	directory used depends on the (g)libc version.
       Libc4 and libc5 use /usr/lib/zoneinfo, and, since libc-5.4.6, when this
       doesn't	work, will try /usr/share/zoneinfo.  Glibc2 will use the envi-
       ronment variable TZDIR, when that exists.  Its default depends  on  how
       it was installed, but normally is /usr/share/zoneinfo.

       This timezone directory contains the files
       localtime      local time zone file
       posixrules     rules for POSIX-style TZ's

       Often  /etc/localtime is a symlink to the file localtime or to the cor-
       rect time zone file in the system time zone directory.

CONFORMING TO
       SVr4, POSIX.1-2001, 4.3BSD

NOTES
       Note that the variable daylight does not indicate that daylight savings
       time  applies  right  now. It used to give the number of some algorithm
       (see the variable tz_dsttime in gettimeofday(2)).  It has been obsolete
       for many years but is required by SUSv2.

       4.3BSD  had a function char *timezone(zone, dst) that returned the name
       of the time zone corresponding to its first argument (minutes  West  of
       GMT).  If the second argument was 0, the standard name was used, other-
       wise the daylight savings time version.

SEE ALSO
       date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)



				  2001-11-13			      TZSET(3)
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