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.Dd December 25, 2023
.Dt TZSET 3
.Os
.Sh NAME
.Nm daylight ,
.Nm timezone ,
.Nm tzname ,
.Nm tzset
.Nd initialize time conversion information
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Vt extern int daylight ;
.Vt extern long timezone ;
.Vt extern char *tzname[2] ;
.Ft void
.Fn tzset void
.Sh DESCRIPTION
The
.Fn tzset
function
initializes time conversion information used by the library routine
.Xr localtime 3 .
The environment variable
.Ev TZ
specifies how this is done.
.Pp
If
.Ev TZ
does not appear in the environment, the best available approximation to
local wall clock time, as specified by the
.Xr tzfile 5 Ns -format
file
.Pa /etc/localtime
is used.
.Pp
If
.Ev TZ
appears in the environment but its value is a null string, Coordinated
Universal Time
.Pq Tn UTC
is used (without leap second correction).
.Pp
If
.Ev TZ
appears in the environment and its value begins with a colon
.Pq Ql \&: ,
the rest of its value is used as a pathname of a
.Xr tzfile 5 Ns -format
file from which to read the time conversion information.
If the first character of the pathname is a slash
.Pq Ql /
it is used as
an absolute pathname; otherwise, it is used as a pathname relative to
the system time conversion information directory.
.Pp
If its value does not begin with a colon, it is first used as the pathname
of a file (as described above) from which to read the time conversion
information.
If that file cannot be read, the value is then interpreted as a direct
specification (the format is described below) of the time conversion
information.
.Pp
If the
.Ev TZ
environment variable does not specify a
.Xr tzfile 5 Ns -format
file and cannot be interpreted as a direct specification,
.Tn UTC
is used.
.Pp
After the first call to
.Nm tzset ,
the
.Vt timezone
variable is set to the difference, in seconds, between Coordinated Universal
Time (UTC) and the local time.
The
.Vt daylight
variable is set to 0 if the local timezone is observing standard time.
It is set to 1 if the local timezone ever observes an alternate time, such as summer time.
The first element of the
.Vt tzname
array is the timezone name of standard time, while the second element is the
name of the altnerative time zone.
.Sh SPECIFICATION FORMAT
When
.Ev TZ
is used directly as a specification of the time conversion information,
it must have the following syntax (spaces inserted for clarity):
.Bd -ragged -offset indent
.Em std offset
.Bo
.Em dst
.Bq Em offset
.Bq , Em rule
.Bc
.Ed
.Pp
Where:
.Bl -tag -width std_and_dst -offset indent
.It Em std No and Em dst
Three or more bytes that are the designation for the standard
.Pq Em std
or summer
.Pq Em dst
time zone.
Only
.Em std
is required; if
.Em 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
.Pq Ql \&: ,
digits, comma
.Pq Ql \&, ,
minus
.Pq Ql \- ,
plus
.Pq Ql + ,
and
.Tn ASCII
.Dv NUL
are allowed.
.It Em offset
Indicates the value one must add to the local time to arrive at
Coordinated Universal Time.
The
.Em offset
has the form:
.Bd -ragged -offset indent
.Sm off
.Em hh Bo
.Em : mm
.Bq Em : ss
.Bc
.Sm on
.Ed
.Pp
The minutes
.Pq Em mm
and seconds
.Pq Em ss
are optional.
The hour
.Pq Em hh
is required and may be a single digit.
The
.Em offset
following
.Em std
is required.
If no
.Em offset
follows
.Em dst ,
summer 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
.Pq Ql \-
the time zone shall be east of the Prime Meridian; otherwise it shall be
west (which may be indicated by an optional preceding
.Pq Ql + ) .
.It Em rule
Indicates when to change to and back from summer time.
The
.Em rule
has the form:
.Bd -ragged -offset indent
.Em date/time,date/time
.Ed
.Pp
where the first
.Em date
describes when the change from standard to summer time occurs and the
second
.Em date
describes when the change back happens.
Each
.Em time
field describes when, in current local time, the change to the other
time is made.
.Pp
The format of
.Em date
is one of the following:
.Bl -tag -width "M.m.n.d"
.It Sy J Em n
The Julian day
.Em n
(1 \*(Le
.Em n
\*(Le 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 Em n
The zero-based Julian day
(0 \*(Le
.Em n
\*(Le 365 ) .
Leap days are counted, and it is possible to refer to February 29.
.It Sy M Em m.n.d
The
.Em d Ns 'th
day (0 \*(Le
.Em d
\*(Le 6)
of week
.Em n
of month
.Em m
of the year
(1 \*(Le
.Em n
\*(Le 5),
(1 \*(Le
.Em m
\*(Le 12),
where week 5 means
.Do
the last
.Em d
day in month
.Em m
.Dc
which may occur in either the fourth or the fifth week).
Week 1 is the
first week in which the
.Em d Ns 'th
day occurs.
Day zero is Sunday.
.Pp
The
.Em time
has the same format as
.Em offset
except that no leading sign
.Pq Ql \-
or
.Pq Ql +
is allowed.
The default, if
.Em time
is not given, is
.Sy 02:00:00 .
.El
.Pp
If no
.Em rule
is present in the
.Ev TZ
specification, the rules specified
by the
.Xr tzfile 5 Ns -format
file
.Em posixrules
in the system time conversion information directory are used, with the
standard and summer time offsets from
.Tn UTC
replaced by those specified by
the
.Em offset
values in
.Ev TZ .
.El
.Pp
For compatibility with System V Release 3.1, a semicolon
.Pq Ql \&;
may be used to separate the
.Em rule
from the rest of the specification.
.Sh FILES
.Bl -tag -width /usr/share/zoneinfo/posixrules -compact
.It Pa /etc/localtime
local time zone file
.It Pa /usr/share/zoneinfo
time zone directory
.It Pa /usr/share/zoneinfo/posixrules
rules for
.Tn POSIX Ns -style
.Tn TZ Ns 's
.It Pa /usr/share/zoneinfo/Etc/GMT
for
.Tn UTC
leap seconds
.El
.Pp
If the file
.Pa /usr/share/zoneinfo/UTC
does not exist,
.Tn UTC
leap seconds are loaded from
.Pa /usr/share/zoneinfo/posixrules .
.Sh SEE ALSO
.Xr date 1 ,
.Xr gettimeofday 2 ,
.Xr ctime 3 ,
.Xr getenv 3 ,
.Xr time 3 ,
.Xr tzfile 5
.Sh HISTORY
The
.Fn tzset
function first appeared in
.Bx 4.4 .