TZ(5) File Formats Manual TZ(5)
NAME
TZ - Time zone environment variable
SYNOPSIS
TZ=zone[-]offset[dst[offset][,start[/time],end[/time]]]
DESCRIPTION
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 day-
light savings 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 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.
start/time,end/time
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 ignored.
Mm.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 Septem-
ber.
The time field indicates the time the changeover occurs on the given day.
EXAMPLES
Greenwich Mean Time:
TZ=GMT0
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:
TZ='MET-1MET DST,M3.5.0/2,M10.5.0/3'
British time, daylight savings starts and ends at the same moment as MET, but in an earlier time zone:
TZ=GMT0BST,M3.5.0/1,M10.5.0/2
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:
TZ=EST5EDT,M4.1.0/2,M10.5.0/2
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:
TZ=NZST-12NZDT,M10.1.0/2,M3.3.0/3
SEE ALSO
readclock(8), date(1).
BUGS
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.
AUTHOR
Kees J. Bot (kjb@cs.vu.nl)
TZ(5)