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tzset - initialize time conversion information
void tzset (void);
extern char *tzname
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.
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
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 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:
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
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
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 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.
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
date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)