Linux man page for tzset.3

tzset(3) -- Linux man page

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 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 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 algorithm (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.