path: root/static/openbsd/man3/tzset.3

.\"	$OpenBSD: tzset.3,v 1.27 2025/06/23 13:53:11 millert Exp $
.Dd $Mdocdate: June 23 2025 $
.Dt TZSET 3
.Os
.Sh NAME
.Nm tzset ,
.Nm tzsetwall
.Nd initialize time conversion information
.Sh SYNOPSIS
.In time.h
.Vt extern char *tzname[2];
.Vt extern long  timezone;
.Vt extern long  daylight;
.Ft void
.Fn tzset "void"
.Ft void
.Fn tzsetwall "void"
.Sh DESCRIPTION
The
.Fn tzset
function uses the value of the environment variable
.Ev TZ
to set the time conversion information used by
.Xr localtime 3 .
It also sets the following external variables:
.Bl -tag -width "tzname[2]"
.It Vt tzname[2]
the designations for standard and daylight saving time; see the description of
.Ar std No and Ar dst
below
.It Vt timezone
the number of seconds west of the Prime Meridian
.It Vt daylight
0 if the time zone has never observed daylight saving time, otherwise
non-zero
.El
.Pp
Most programs do not need to call
.Fn tzset
directly; it will be called automatically as needed by the functions
described in
.Xr localtime 3 .
Privileged processes that use
.Xr chroot 2
may wish to call
.Fn tzset
to initialize the time conversion information before changing to
a restricted root directory that does not include time conversion
data files.
.Pp
If
.Ev TZ
does not appear in the environment, or if the calling process has
changed its user or group ID, the system time zone file,
.Pa /etc/localtime ,
is used.
.Pp
If
.Ev TZ
appears in the environment it may be one of two formats:
.Bl -bullet
.It
the pathname of a
.Xr tzfile 5
format file from which to read the time conversion information,
optionally prefixed with a colon
.Pq Ql \&: ,
such as
.Dq :America/Denver
or
.Dq Europe/Berlin
.It
a string that directly specifies the time conversion information
(see below) which may not begin with a colon
.Pq Ql \&:
.El
.Pp
If
.Ev TZ
appears in the environment and its value does not begin with a colon,
it is first used as the
pathname of a
.Xr tzfile 5
format 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.
A value beginning with a colon
.Pq Ql \&:
is always treated as a pathname.
.Pp
If
.Ev TZ
is set to the empty string, Universal Time (UT) is used, with the abbreviation
.Dq UTC
and without leap second correction; please see
.Xr ctime 3
for more about UT, UTC, and leap seconds.
.Pp
When
.Ev TZ
is used as a pathname, it must either be a path relative to the system time
conversion information directory,
.Pa /usr/share/zoneinfo ,
or an absolute path that begins with
.Pa /usr/share/zoneinfo/ .
Other absolute paths, or paths that contain
.Ql \&../ ,
will be ignored and the system local time zone file,
.Pa /etc/localtime ,
will be used instead.
The file must be in the format specified in
.Xr tzfile 5 .
.Pp
When
.Ev TZ
is used directly as a specification of the time conversion information,
it must have the following syntax (without whitespace between
.Ar std
and
.Ar offset ) :
.Bd -ragged -offset indent
.Ar std
.Sm off
.Ar offset
.Op Ar dst Op Ar offset
.Op , Ar rule
.Sm on
.Ed
.Pp
Where:
.Bl -tag -width "std and dst"
.It Ar std No and Ar dst
Three or more bytes that are the designation for the standard
.Pq Ar std
or the daylight saving
.Pq Ar dst
time zone.
Only
.Ar std
is required; if
.Ar dst
is missing, then daylight saving time does not apply in this locale.
Upper and lowercase letters are explicitly allowed.
Any characters except a leading colon
.Pq Ql \&: ,
digits, comma
.Pq Ql \&, ,
minus
.Pq Ql \&- ,
plus
.Pq Ql \&+ ,
and ASCII NUL are allowed.
.It Ar offset
Indicates the value one must add to the local time to arrive at
Coordinated Universal Time.
.Ar offset
has the form:
.Pp
.D1 Ar hh Ns Op : Ns Ar mm Ns Op : Ns Ar ss
.Pp
The minutes
.Pq Ar mm
and seconds
.Pq Ar ss
are optional.
The hour
.Pq Ar hh
is required and may be a single digit.
The
.Ar offset
following
.Ar std
is required.
If no
.Ar offset
follows
.Ar dst ,
daylight saving time is assumed to be one hour ahead of standard time.
One or more digits may be used; the value is always interpreted as a
decimal number.
The hour must be between zero and 24, and the minutes (and
seconds) \(em if present \(em between zero and 59.
If preceded by a
.Dq \&- ,
the time zone shall be east of the Prime Meridian; otherwise it shall be
west (which may be indicated by an optional preceding
.Dq \&+ ) .
.It Ar rule
Indicates when to change to and back from daylight saving time.
.Ar rule
has the form:
.Pp
.D1 Ar date Ns / Ns Ar time , Ns Ar date Ns / Ns Ar time
.Pp
where the first
.Ar date
describes when the change from standard to daylight saving time occurs and the
second
.Ar date
describes when the change back happens.
Each
.Ar time
field describes when, in current local time, the change to the other
time is made.
As an extension to POSIX, daylight saving is assumed to be in effect
all year if it begins January 1 at 00:00 and ends December 31 at
24:00 plus the difference between daylight saving and standard time,
leaving no room for standard time in the calendar.
.Pp
The format of
.Ar date
is one of the following:
.Bl -tag -width Ds
.It Cm J Ns Ar n
The Julian day
.Ar n
.Pq 1 <= Ar n No <= 365 .
Leap days are not counted; that is, in all years \(em including leap
years \(em February 28 is day 59 and March 1 is day 60.
It is impossible to explicitly refer to the occasional February 29.
.It Ar n
The zero-based Julian day
.Pq 0 <= Ar n No <= 365 .
Leap days are counted, and it is possible to refer to February 29.
.It Cm M Ns Ar m . Ns Ar n . Ns Ar d
Day
.Ar d
.Pq 1 <= Ar d No <= 6
of week
.Ar n
.Pq 1 <= Ar n No <= 5
of month
.Ar m
.Pq 1 <= Ar m No <= 12 ,
where week 5 means
.Do
the last
.Ar d
day in month
.Ar m
.Dc
which may occur in either the fourth or the fifth week.
Week 1 is the first week in which the
.Ar d Ns th
day occurs.
Day zero is Sunday.
.El
.Pp
The
.Ar time
has the same format as
.Ar offset
except that POSIX does not allow a leading sign
.Po
.Dq \&-
or
.Dq \&+
.Pc .
As an extension to POSIX, the hours part of
.Ar time
can range from \-167 through 167; this allows for unusual rules such as
.Dq the Saturday before the first Sunday of March .
The default, if
.Ar time
is not given, is
.Cm 02:00:00 .
.El
.Pp
Here are some examples of
.Ev TZ
values that directly specify the time zone rules;
they use some of the extensions to POSIX.
.Bl -tag -width Ds
.It EST5
stands for US Eastern Standard Time (EST),
5 hours behind UT, without daylight saving.
.It FJT\-12FJST,M10.3.1/146,M1.3.4/75
stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours ahead
of UT, springing forward on October's third Monday at
146:00 (i.e., 02:00 on the first Sunday on or after October 21), and
falling back on January's third Thursday at 75:00 (i.e., 03:00 on the
first Sunday on or after January 18).
.It IST\-2IDT,M3.4.4/26,M10.5.0
stands for Israel Standard Time (IST) and Israel Daylight Time (IDT),
2 hours ahead of UT, springing forward on March's fourth
Tuesday at 26:00 (i.e., 02:00 on the first Friday on or after March
23), and falling back on October's last Sunday at 02:00.
.It WART4WARST,J1/0,J365/25
stands for Western Argentina Summer Time (WARST), 3 hours behind UT.
There is a dummy fall-back transition on December 31 at 25:00 daylight
saving time (i.e., 24:00 standard time, equivalent to January 1 at
00:00 standard time), and a simultaneous spring-forward transition on
January 1 at 00:00 standard time, so daylight saving time is in effect
all year and the initial
.Dq WART
is a placeholder.
.It WGT3WGST,M3.5.0/\-2,M10.5.0/\-1
stands for Western Greenland Time (WGT) and Western Greenland Summer
Time (WGST), 3 hours behind UT, where clocks follow the EU rules of
springing forward on March's last Sunday at 01:00 UT (\-02:00 local
time) and falling back on October's last Sunday at 01:00 UT
(\-01:00 local time).
.Pp
If no
.Ar rule
is present in
.Ev TZ ,
the rules specified
by the
.Xr tzfile 5
format
file
.Cm posixrules
in the system time conversion information directory are used, with the
standard and daylight saving time offsets from UT replaced by those
specified by the
.Ar offset
values in
.Ev TZ .
.Pp
For compatibility with System V Release 3.1, a semicolon
.Pq Ql \&;
may be used to separate the
.Ar rule
from the rest of the specification.
.Pp
If the
.Ev TZ
environment variable does not specify a
.Xr tzfile 5
format file
and cannot be interpreted as a direct specification,
UT is used.
.Pp
.Fn tzsetwall
behaves identically to
.Fn tzset
but it only uses the
.Pa /etc/localtime
file (that is, it ignores the
.Ev TZ
environment variable).
.Sh FILES
.Bl -tag -width "/usr/share/zoneinfo/posixrules" -compact
.It Pa /usr/share/zoneinfo
time zone information directory
.It Pa /etc/localtime
local time zone file
.It Pa /usr/share/zoneinfo/posixrules
used with POSIX-style
.Ev TZ Ns s
.It Pa /usr/share/zoneinfo/GMT
for UTC leap seconds
.El
.Pp
If
.Pa /usr/share/zoneinfo/GMT
is absent,
UTC leap seconds are loaded from
.Pa /usr/share/zoneinfo/posixrules .
.Sh SEE ALSO
.Xr ctime 3 ,
.Xr getenv 3 ,
.Xr strftime 3 ,
.Xr time 3 ,
.Xr tzfile 5
.Sh STANDARDS
The
.Fn tzset
function
conforms to
.St -p1003.1-2008 .
The
.Fn tzsetwall
function is an extension to that specification.
.\" This file is in the public domain, so clarified as of
.\" 2009-05-17 by Arthur David Olson.