tzset

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

#include <time.h>
 void tzset (void);
 I extern char * tzname [2];
I extern long  timezone ;
I extern int  daylight ;

The R 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 saving time rules, non-zero if there is a time during the year when daylight saving 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 empty 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 second format is used when there is daylight saving time:

The start field specifies when daylight saving 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

n

Mm.w.d

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. Here is an example for New Zealand, where the standard time (NZST) is 12 hours ahead of UTC, and daylight saving time (NZDT), 13 hours ahead of UTC, runs from the first Sunday in October to the third Sunday in March, and the changeovers happen at the default time of 02:00:00:

    TZ="NZST-12.00:00NZDT-13:00:00,M10.1.0,M3.3.0"

The third format specifies that the time zone information should be read from a file:

Here's an example, once more for New Zealand:

    TZ=":Pacific/Auckland"

The system time zone directory used depends on the (g)libc version. Libc4 and libc5 use R /usr/lib/zoneinfo , and, since libc-5.4.6, when this doesn't work, will try R /usr/share/zoneinfo . Glibc2 will use the environment variable R TZDIR , when that exists. Its default depends on how it was installed, but normally is R /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.

SVr4, POSIX.1-2001, 4.3BSD

Note that the variable daylight does not indicate that daylight saving 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.

4.3BSD had a function I 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 saving time version.

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