-.\" $OpenBSD: tzset.3,v 1.25 2022/09/23 17:29:22 millert Exp $
-.Dd $Mdocdate: September 23 2022 $
+.\" $OpenBSD: tzset.3,v 1.26 2022/10/04 13:33:57 millert Exp $
+.Dd $Mdocdate: October 4 2022 $
.Dt TZSET 3
.Os
.Sh NAME
.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
-uses the value of the environment variable
+function uses the value of the environment variable
.Ev TZ
-to set time conversion information used by
+to set the time conversion information used by
.Xr localtime 3 .
-If
-.Ev TZ
-does not appear in the environment,
-or if the calling process has changed its user or group ID,
-the best available approximation to local wall clock time, as specified
-by the
-.Xr tzfile 5
-format file
-.Pa /etc/localtime ,
-is used by
+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 UTC
+.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
-appears in the environment but its value is a null string,
-Coordinated Universal Time (UTC) is used (without leap second
-correction).
+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 and its value begins with a colon,
-it is used as the pathname of a file
-from which to read the time conversion information.
+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 file from which to read the time conversion information,
+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, Coordinated Universal Time (UTC) is
+used (without leap second correction).
.Pp
When
.Ev TZ
-is used as a pathname, it must be relative to the system time
+is used as a pathname, it must either be a path relative to the system time
conversion information directory,
-.Pa /usr/share/zoneinfo .
-If
-.Ev TZ
-begins with a
-.Ql /
-or contains
+.Pa /usr/share/zoneinfo ,
+or an absolute path that begins with
+.Pa /usr/share/zoneinfo/ .
+Other absolute paths, or paths that contain
.Ql \&../ ,
-it is ignored and the system local time zone file,
+will be ignored and the system local time zone file,
.Pa /etc/localtime ,
-is used instead.
+will be used instead.
The file must be in the format specified in
.Xr tzfile 5 .
.Pp
.It Ar std No and Ar dst
Three or more bytes that are the designation for the standard
.Pq Ar std
-or summer
+or the daylight saving
.Pq Ar dst
time zone.
Only
.Ar std
is required; if
.Ar dst
-is missing, then summer time does not apply in this locale.
+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 Sq \&: ,
+.Pq Ql \&: ,
digits, comma
-.Pq Sq \&, ,
+.Pq Ql \&, ,
minus
-.Pq Sq \&- ,
+.Pq Ql \&- ,
plus
-.Pq Sq \&+ ,
+.Pq Ql \&+ ,
and ASCII NUL are allowed.
.It Ar offset
Indicates the value one must add to the local time to arrive at
.Ar offset
follows
.Ar dst ,
-summer time is assumed to be one hour ahead of standard time.
+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
west (which may be indicated by an optional preceding
.Dq \&+ ) .
.It Ar rule
-Indicates when to change to and back from summer time.
+Indicates when to change to and back from daylight saving time.
.Ar rule
has the form:
.Pp
.Pp
where the first
.Ar date
-describes when the change from standard to summer time occurs and the
+describes when the change from standard to daylight saving time occurs and the
second
.Ar date
describes when the change back happens.
file
.Cm posixrules
in the system time conversion information directory are used, with the
-standard and summer time offsets from UTC replaced by those specified by
-the
+standard and daylight saving time offsets from UTC replaced by those
+specified by the
.Ar offset
values in
.Ev TZ .
.Pp
For compatibility with System V Release 3.1, a semicolon
-.Pq Sq \&;
+.Pq Ql \&;
may be used to separate the
.Ar rule
from the rest of the specification.
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