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

NAME

       tzset, tzname, timezone, daylight - initialize time conversion information

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 environment 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 savings time rules, nonzero if there is a time during the year when
       daylight savings time applies).

       If the TZ variable does not appear in the environment, the tzname variable 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 NULL or its value cannot be interpreted using any of the formats  speci-
       fied 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 Coordinated 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 savings 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.  February 29 is never counted even in leap years.

       n      This specifies the Julian day with n between 1 and 365.  February 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 information 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 filespec 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 environment 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 correct time zone file in the system time zone directory.

CONFORMING TO

       SVID 3, POSIX, BSD 4.3

NOTES

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

       BSD4.3  had a routine 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, otherwise the daylight savings time version.

SEE ALSO

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

								    2001-11-13								  TZSET(3)