tzset - initialize time conversion information
tzset() uses the value of the environment variable TZ to set
information used by localtime(3). If TZ does not appear in the environment,
the best available approximation to local wall
clock time, as
specified by the tzfile(5) format file localtime in the system time conversion
information directory, is used by localtime(3).
If TZ appears in the environment but its value is a null
Universal Time (UTC) is used (without leap second correction).
If TZ appears in the environment and its value begins with a
colon, it is
used as a pathname of a file from which to read the time
If TZ appears in the environment and its value does not begin with a
colon, it is first used as the pathname of a file from which
to read the
time conversion information, and, if that file cannot be
read, is used
directly as a specification of the time conversion information.
When TZ is used as a pathname, if it begins with a slash, it
is used as
an absolute pathname; otherwise, it is used as a pathname
relative to a
system time conversion information directory. The file must
be in the
format specified in tzfile(5).
When TZ is used directly as a specification of the time conversion information,
it must have the following syntax (spaces inserted
std offset [dst [offset]] [, rule]
std and dst Three or more bytes that are the designation
for the standard
(std) or summer (dst) time zone. Only std
if dst is missing, then summer time does not
apply in this
locale. Upper and lowercase letters are explicitly allowed.
Any characters except a leading colon (`:'),
(`,'), minus (`-'), plus (`+'), and ASCII NUL
offset Indicates the value one must add to the local
time to arrive
at Coordinated Universal Time. offset has the
inserted for clarity):
hh [: mm [: ss]]
The minutes (mm) and seconds (ss) are optional.
(hh) is required and may be a single digit.
The offset following
std is required. If no offset follows
time is assumed to be one hour ahead of standard time. One
or more digits may be used; the value is always
as a decimal number. The hour must be between
zero and 24,
and the minutes (and seconds) -- if present --
and 59. If preceded by a ``-'', the time zone
shall be east
of the Prime Meridian; otherwise it shall be
west (which may
be indicated by an optional preceding ``+'').
rule Indicates when to change to and back from summer time. rule
has the form (spaces added for clarity):
date / time, date / time
where the first date describes when the change
to summer time occurs and the second date describes when the
change back happens. Each time field describes
current local time, the change to the other
time is made.
The format of date is one of the following
(spaces added for
J n The Julian day n (1 <= n <= 365).
are not counted; that is, in all
years -- including
leap years -- February 28
is day 59 and
March 1 is day 60. It is impossible to explicitly
refer to the occasional
n The zero-based Julian day (0 <= n
Leap days are counted, and it is
refer to February 29.
M m . n . d Day d (1 <= d <= 6) of week n (1
<= n <= 5) of
month m (1 <= m <= 12), where week
``the last d day in month m''
which may occur
in either the fourth or the fifth
week. Week 1
is the first week in which the dth
Day zero is Sunday.
The time has the same format as offset except
that no leading
sign (``-'' or ``+'') is allowed. The default, if time
is not given, is 02:00:00.
If no rule is present in TZ, the rules specified by the tzfile(5) format
file posixrules in the system time conversion information
used, with the standard and summer time offsets from UTC replaced by
those specified by the offset values in TZ.
For compatibility with System V Release 3.1, a semicolon
(`;') may be
used to separate the rule from the rest of the specification.
If the TZ environment variable does not specify a tzfile(5)
and cannot be interpreted as a direct specification, UTC is
/usr/share/zoneinfo time zone information directory
/etc/localtime local time zone file
/usr/share/zoneinfo/posixrules used with POSIX-style TZs
/usr/share/zoneinfo/GMT for UTC leap seconds
If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are
ctime(3), getenv(3), strftime(3), time(3), tzfile(5)
OpenBSD 3.6 May 24, 1999
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