.\"	$NetBSD: tzset.3,v 1.46 2023/09/16 18:40:26 christos Exp $
.\" This file is in the public domain, so clarified as of
.\" 2009-05-17 by Arthur David Olson.
.Dd September 16, 2023
.Dt TZSET 3
.Os
.Sh NAME
.Nm tzset ,
.Nm tzalloc ,
.Nm tzgetname ,
.Nm tzgetgmtoff ,
.Nm tzfree
.Nd initialize time conversion information
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Ft timezone_t
.Fn tzalloc "const char *zone"
.Ft void
.Fn tzfree "timezone_t restrict tz"
.Ft const char *
.Fn tzgetname "timezone_t restrict tz" "int isdst"
.Ft long
.Fn tzgetgmtoff "timezone_t restrict tz" "int isdst"
.Ft void
.Fn tzset "void"
.Sh DESCRIPTION
The
.Fn tzalloc
function takes as an argument a timezone name and returns a
.Ft timezone_t
object suitable to be used in the
.Fn ctime_rz ,
.Fn localtime_rz ,
and
.Fn mktime_z
functions.
.Pp
If
.Ar zone
is not a valid timezone description, or if the object cannot be allocated,
.Fn tzalloc
returns a
.Dv NULL
pointer and sets
.Va errno .
.Pp
A
.Dv NULL
pointer may be passed to
.Fn tzalloc
instead of a timezone name, to refer to the current system timezone.
An empty timezone string indicates Coordinated Universal Time
.Pq Tn UTC .
.Pp
Note that instead of setting the environment variable
.Va TZ ,
and globally changing the behavior of the calling program, one can use
multiple timezones at the same time by using separate
.Ft timezone_t
objects allocated by
.Fn tzalloc
and calling the
.Dq z
variants of the functions.
The
.Fn tzfree
function deallocates
.Fa tz ,
which was previously allocated by
.Fn tzalloc .
This invalidates any
.Ft tm_zone
pointers that
.Fa tz
was used to set.
The function
.Fn tzgetname
returns the name for the given
.Fa tz .
If
.Fa isdst
is
.Va 0 ,
the call is equivalent to
.Va tzname[0] .
If
.Fa isdst
is set to
.Va 1
the call is equivalent to
.Va tzname[1] .
The return values for both
.Fn tzgetname
and
.Fn tzgmtoff
correspond to the latest time for which data is available, even if that
refers to a future time.
Finally, the
.Fn tzgetgmtoff
function acts like
.Fn tzgetname
only it returns the offset in seconds from GMT for the timezone.
If there is no match, then
.Dv \-1
is returned and
.Va errno
is set to
.Er ESRCH .
The
.Fn tzset
function acts like
.Dv tzalloc(getenv("TZ")) ,
except it saves any resulting timezone object into internal
storage that is accessed by
.Fn localtime ,
.Fn localtime_r ,
and
.Fn mktime .
The anonymous shared timezone object is freed by the next call to
.Fn tzset .
If the implied call to
.Fn tzalloc
fails,
.Fn tzset
falls back on Universal Time (UT).
If
.Ev TZ
is
.Dv NULL ,
the best available approximation to local (wall clock) time, as
specified by the
.Xr tzfile 5
format file
.Pa /etc/localtime
is used by
.Xr localtime 3 .
If
.Ev TZ
appears in the environment but its value is the empty string,
UT is used, with the abbreviation
.Dq UTC
and without leap second correction; please see
.Xr ctime 3 .
If
.Ev TZ
is nonnull and nonempty:
.Bl -dash
.It
if the value begins with a colon, it is used as a pathname of a file
from which to read the time conversion information;
.It
if the 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.
.El
.Pp
When
.Ev 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
.Pa /usr/share/zoneinfo .
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 (spaces inserted for clarity):
.Sm off
.Bd -literal -offset indent
.Cm std Cm offset Oo Cm dst Oo Cm offset Oc Oo , Cm rule Oc Oc
.Ed
.Sm on
.Pp
where:
.Bl -tag -width "std and dst" -compact
.It Cm std No and Cm dst
Three or more bytes that are the designation for the standard
.Cm ( std )
or the alternative
.Cm ( dst
such as daylight saving time)
timezone.
Only
.Cm std
is required; if
.Cm 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 (:), digits, comma (,), minus (-),
plus (+), and NUL bytes are allowed.
Alternatively, a designation can be surrounded by angle brackets
.Dv <
and
.Dv > ;
in this case, the designation can contain any characters other than
.Dv >
and
.Dv NUL .
.It Cm offset
Indicates the value one must add to the local time to arrive at
Coordinated Universal Time.
The
.Cm offset
has the form:
.Sm off
.Bd -literal -offset indent
.Cm hh Oo
.Cm :mm Oo
.Cm :ss Oc Oc
.Ed
.Sm on
.Pp
The minutes
.Cm ( mm )
and seconds
.Cm ( ss )
are optional.
The hour
.Cm ( hh )
is required and may be a single digit.
The
.Cm offset
following
.Cm std
is required.
If no
.Cm offset
follows
.Cm 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) \(en if present \(en between zero and 59.
If preceded by a
.Dq -
the timezone shall be east of the Prime Meridian; otherwise it shall be
west (which may be indicated by an optional preceding
.Dq + ) .
.It Cm rule
Indicates when to change to and back from daylight saving time.
The
.Cm rule
has the form:
.Sm off
.Bd -literal -offset indent
.Xo
.Cm date No /
.Cm time ,
.Cm date No /
.Cm time
.Xc
.Ed
.Sm on
.Pp
where the first
.Cm date
describes when the change from standard to daylight saving time occurs and the
second
.Cm date
describes when the change back happens.
Each
.Cm 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.
The format of
.Fa date
is one of the following:
.Bl -tag -width "The Julian day" -compact
.It Cm J Ns Ar n
The Julian day
.Ar n
(1 \*[Le]
.Ar n
\*[Le] 365).
Leap days are not counted; that is, in all years \(en including leap
years \(en 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 (0\ \*[Le]
.Ar n
\*[Le]\ 365).
Leap days are counted, and it is possible to refer to
February 29.
.Sm off
.It Cm M Ns Ar m . Ar n . Ar d
.Sm on
The
.Ar d Ns 'th
day
(0 \*[Le]
.Ar d
\*[Le]\ 6) of week
.Ar n
of month
.Ar m
of the year
(1 \*[Le]
.Ar n
\*[Le]\ 5, 1 \*[Le]
.Ar m
\*[Le]\ 12, where week 5 means
.Dq the\ last Ar d No day\ in\ month Ar m
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
The
.Cm time
has the same format as
.Cm offset
except that POSIX does not allow a leading sign
.Dq -
or
.Dq +
is allowed.
As an extension to POSIX, the hours part of
.Cm 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
.Cm time
is not given, is
.Cm 02:00:00 .
.El
.Pp
Here are some examples of
.Va TZ
values that directly specify the timezone rules; they use some of the
extensions to POSIX.
.Bl -tag -width 6n
.It EST5
stands for US Eastern Standard
Time (EST), 5 hours behind UT, without daylight saving.
.It FJT\-12FJST,M11.1.0,M1.3.4/75
.It <+12>\-12<+13>,M11.1.0,M1.2.1/147
stands for Fiji time, 12 hours ahead
of UT, springing forward on November's first Sunday at 02:00, and
falling back on January's second Monday at 147:00 (i.e., 03:00 on the
first Sunday on or after January 14).
The abbreviations for standard and daylight saving time are
.Qq +12
and
.Qq +13 .
.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
Thursday 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 <\-04>4<\-03>,J1/0,J365/25
stands for permanent daylight saving time, 3 hours behind UT with
abbreviation
.Qq \-03 .
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
.Em <\-04>
is a placeholder.
.It <\-03>3<\-02>,M3.5.0/\-2,M10.5.0/\-1
stands for time in western Greenland, 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, i.e., 22:00 the previous day) and falling back on October's last
Sunday at 01:00 UT (\-01:00 local time, i.e., 23:00 the previous day).
The abbreviations for standard and daylight saving time are
.Qq \-03
and
.Qq \-02 .
.El
.Pp
If no
.Cm rule
is present in
.Ev TZ ,
the rules specified by the
.Xr tzfile 5
format file
.Pa posixrules
in
.Pa /usr/share/zoneinfo
are used, with the standard and daylight saving time offsets from UT replaced
by those specified by the
.Cm offset
values in
.Ev TZ .
.Pp
For compatibility with System V Release 3.1, a semicolon (;) may be
used to separate the
.Cm rule
from the rest of the specification.
.Sh FILES
.Bl -tag -width /usr/share/zoneinfo/posixrules -compact
.It Pa /etc/localtime
local timezone file
.It Pa /usr/share/zoneinfo
local timezone information directory
.\" .It Pa /usr/share/zoneinfo/localtime
.\" local timezone file
.It Pa /usr/share/zoneinfo/posixrules
default DST rules (obsolete,
and can cause bugs if present)
used with POSIX-style TZ
.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 -p1003.1-88 .
.Sh BUGS
Neither the
.Fn tzgetname
nor
.Fn tzgmtoff
functions have the ability to specify the point in time for which the
requested data should be returned.
.\" @(#)newtzset.3	8.2
.\" This file is in the public domain, so clarified as of
.\" 2009-05-17 by Arthur David Olson.