TZ - Time zone environment variable
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 POSIX standardized form shown in the synopsis.
This form specifies the zone names, offsets from GMT, and daylight
savings changeover times for at least the current year.
zone A three or more letter name for the time zone in normal (winter)
A signed time telling the offset of the time zone westwards from
Greenwich. The time has the form hh[:mm[:ss]] with a one of two
digit hour, and optional two digit minutes and seconds.
dst The name of the time zone when daylight savings is in effect. It
may be followed by an offset telling how big the clock correction is
other than the default of 1 hour.
Specifies the start and end of the daylight savings 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
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
The time field indicates the time the changeover occurs on the given
Greenwich Mean Time:
Middle European Time, 1 hour east from Greenwich, daylight savings starts
on the last Sunday in March at 2 AM and ends on the last Sunday in
October at 3 AM:
British time, daylight savings starts and ends at the same moment as MET,
but in an earlier time zone:
The eastern european time zones also have the changeovers at the same
absolute time as British time and MET.
U.S. Eastern Standard Time, 5 hours west from Greenwich, daylight savings
starts on the first Sunday in April at 2 AM and ends on the last Sunday
in October at 2 AM:
It shouldn't surprise you that daylight savings 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:
You may have noticed that many fields are optional. Do no omit them,
because the defaults are bogus. If you need daylight savings then fully
specify the changeovers.
West is negative, east is positive, ask any sailor.
Kees J. Bot (firstname.lastname@example.org)