tzset - initialize time conversion information
void tzset (void);
extern char *tzname
The 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.
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 /usr/lib/zoneinfo/localtime.
If the TZ variable does appear in the environment but its
value is NULL 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 for-
mat is used when there is no daylight saving time in the
local time zone:
The std 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). The offset 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
The second format is used when there is daylight saving
std offset dst [offset],start[/time],end[/time]
There are no spaces in the specification. The initial std
and offset 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
The start 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 fol-
Jn This specifies the Julian day with n between 1 and 365.
February 29 is never counted even in leap years.
n This specifies the Julian day with n between 1 and 365.
February 29 is counted in leap years.
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 /usr/lib/zoneinfo/localtime which
is in tzfile(5) format. If filespec is given, it specifies
another tzfile(5)-format file to read the time zone informa-
tion from. If filespec does not begin with a `/', the file
specification is relative to the system time conversion
information directory /usr/lib/zoneinfo.
/usr/lib/zoneinfo system time zone directory
/usr/lib/zoneinfo/localtime local time zone file
/usr/lib/zoneinfo/posixrules rules for POSIX-style TZ's
SVID 3, POSIX, BSD 4.3
date(1), gettimeofday(2), time(2), getenv(3), tzfile(5)