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RedHat 9 (Linux i386) - man page for tzset (redhat section 3)

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

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

       #include <time.h>

       void tzset (void);

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

       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

       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 can-
       not be interpreted using any of the formats specified 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  fol-
       lowing 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:


       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  speci-
       fies  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

       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.

       SVID 3, POSIX, BSD 4.3

       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 gettime-
       ofday(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 cor-
       responding 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.

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

					    2001-11-13					 TZSET(3)

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