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, non-zero 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-format
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:
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
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
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.
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:
|J n||This specifies the Julian day with n between 1 and 365. February 29 is never counted even in leap years.
||This specifies the Julian day with n between 1 and 365. February 29 is counted in leap years.
|M m.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:
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
format. If filespec
is given, it specifies another tzfile-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.