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tzset(3) - daylight, timezone, tzname, tzset, daylight, timezone, tzname, tzset - initialize time conversion information - man 3 tzset

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



NAME
       tzset, tzname, timezone, daylight - initialize time(1,2,n) 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(1,2,n) conversion functions that depend on the time(1,2,n) zone.  In a SysV-like
       environment  it  will  also set(7,n,1 builtins) the variables timezone (seconds West of
       GMT) and daylight (0 if(3,n) this time(1,2,n) zone does not have any daylight  sav-
       ings  time(1,2,n)  rules, nonzero if(3,n) there is a time(1,2,n) during the year when day-
       light savings time(1,2,n) applies).

       If the TZ variable does not appear in(1,8) the environment, the tzname vari-
       able  is  initialized  with  the best approximation of local wall clock(3,n)
       time(1,2,n), as specified by the tzfile(5)-format file(1,n) localtime found in(1,8)  the
       system   timezone   directory   (see  below).   (One  also  often  sees
       /etc/localtime used here, a symlink to the right  file(1,n)  in(1,8)  the  system
       timezone directory.)

       If the TZ variable does appear in(1,8) the environment but its value is NULL
       or its value cannot 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(1,2,n) in(1,8) the local time(1,2,n) zone:

              std offset

       The std string(3,n) specifies the name of the time(1,2,n) zone and must be three or
       more  alphabetic characters.  The offset string(3,n) immediately follows std
       and specifies the time(1,2,n) value to be added to the local time(1,2,n) to get Coor-
       dinated Universal Time (UTC).  The offset is positive if(3,n) the local time(1,2,n)
       zone is west of the Prime Meridian and negative if(3,n)  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(1,2,n)],end[/time(1,2,n)]

       There  are  no spaces in(1,8) the specification.  The initial std and offset
       specify the standard time(1,2,n) zone, as described above.  The dst string(3,n) and
       offset  specify the name and offset for the corresponding daylight sav-
       ings time(1,2,n) zone.  If the offset is omitted, it  defaults   to  one  hour
       ahead of standard time.

       The  start  field specifies when daylight savings time(1,2,n) 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(1,8) leap years.

       n      This specifies the Julian day with n between 1 and 365.   Febru-
              ary 29 is counted in(1,8) 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(1,8) which day d
              occurs and week 5 is the last week in(1,8) which day d occurs.  Day 0
              is a Sunday.

       The time(1,2,n) fields specify when, in(1,8) the local time(1,2,n)  currently  in(1,8)  effect,
       the  change  to  the  other  time(1,2,n)  occurs.   If omitted, the default is
       02:00:00.

       The third format specifies that the time(1,2,n)  zone  information  should  be
       read(2,n,1 builtins) from a file:

              :[filespec]

       If  the  file(1,n) specification filespec is omitted, the time(1,2,n) zone informa-
       tion is read(2,n,1 builtins) from the file(1,n) localtime in(1,8) the system timezone  directory,
       which  nowadays  usually  is  /usr/share/zoneinfo.   This  file(1,n)  is  in(1,8)
       tzfile(5)  format.   If  filespec  is  given,  it   specifies   another
       tzfile(5)-format file(1,n) to read(2,n,1 builtins) the time(1,2,n) zone information from.  If file-
       spec does not begin with a `/', the file(1,n) specification is  relative  to
       the system timezone directory.

FILES
       The  system  time(1,2,n)  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(1,2,n) zone file(1,n)
       posixrules     rules for POSIX-style TZ's

       Often  /etc/localtime is a symlink to the file(1,n) localtime or to the cor-
       rect time(1,2,n) zone file(1,n) in(1,8) the system time(1,2,n) zone directory.

CONFORMING TO
       SVID 3, POSIX, BSD 4.3

NOTES
       Note that the variable daylight does not indicate that daylight savings
       time(1,2,n)  applies  right  now. It used to give the number of some algorithm
       (see the variable tz_dsttime in(1,8) 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(1,2,n) 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(1,2,n) version.

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



                                  2001-11-13                          TZSET(3)

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