tzset - initialize time conversion information
SYNOPSIS
#include <time.h>
void tzset (void);
extern char *tzname[2]
DESCRIPTION
The tzset() function initializes the tzname variable from
the TZ environment variable. This function is automati
cally 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 time
zone 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 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 min
utes 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
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 Sun
day.
The time fields specify when, in the local time currently
in effect, the change to the other time occurs. If omit
ted, 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)-for
mat 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 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
SEE ALSO
date(1), gettimeofday(2), time(2), ctime(3), getenv(3),
tzfile(5)
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