TZ - Time zone environment variable TZ=zone[-]offset[dst[offset][,start[/time],end[/time]]] .. The TZ environment variable tells functions such as the ctime (3) family and programs like date what the time zone and daylight saving rule is. The value of TZ has the \s-2POSIX\s+2 standardized form shown in the synopsis. This form specifies the zone names, offsets from GMT, and daylight saving changeover times for at least the current year.

zone A three or more letter name for the time zone in normal (winter) time.

[-] offset A signed time telling the offset of the time zone westwards from Greenwich. The time has the form hh[:mm[:ss]] with a one or two digit hour, and optional two digit minutes and seconds.

dst The name of the time zone when daylight saving is in effect. It may be followed by an offset telling how big the clock correction is other than the default of 1 hour.

start/time,end/time Specifies the start and end of the daylight saving period. The start and end fields indicate on what day the changeover occurs. They must be in one of the following formats:

Jn The Julian day n (1 <= n <= 365) ignoring leap days, i.e. there is no February 29.

n The zero-based Julian day (0 <= n <= 365). Leap days are not ignored.

M m . n . d

This indicates month m , the n -th occurrence of day d (1 <= m <= 12, 1 <= n <= 5, 0 <= d <= 6, 0=Sunday). The 5-th occurrence means the last occurrence of that day in a month. So M4.1.0 is the first Sunday in April, M9.5.0 is the last Sunday in September.

The time field indicates the time the changeover occurs on the given day.

Greenwich Mean Time:

Central European Time, 1 hour east from Greenwich, daylight saving starts on the last Sunday in March at 2 AM and ends on the last Sunday in October at 3 AM:

British time, daylight saving starts and ends at the same moment as CET, but in an earlier time zone:

The eastern european time zones also have the changeovers at the same absolute time as British time and CET.

U.S. Eastern Standard Time, 5 hours west from Greenwich, daylight saving starts on the second Sunday in March at 2 AM and ends on the first Sunday in November at 2 AM:

It shouldn't surprise you that daylight saving in New Zealand is observed in the months opposite from the previous examples. It starts on the first Sunday in October at 2 AM and ends on the third Sunday in March at 3 AM:

readclock (8), date (1). You may have noticed that many fields are optional. Do not omit them, because the defaults are bogus. If you need daylight saving then fully specify the changeovers.

West is negative, east is positive, ask any sailor.

Next year's time zone and daylight saving time are determined by politicians.

Kees J. Bot (kjb@cs.vu.nl) corrected for US/Canada 2007 changes by ASW 2007-02-20