.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
-.\" %%%LICENSE_START(verbatim)
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\" Modified 2001-11-13, aeb
.\" Modified 2004-12-01 mtk and Martin Schulze <joey@infodrom.org>
.\"
-.TH TZSET 3 2012-03-25 "" "Linux Programmer's Manual"
+.TH TZSET 3 2017-09-15 "" "Linux Programmer's Manual"
.SH NAME
tzset, tzname, timezone, daylight \- initialize time conversion information
.SH SYNOPSIS
.nf
.B #include <time.h>
-.sp
+.PP
.B void tzset (void);
-.sp
+.PP
.BI "extern char *" tzname [2];
.BI "extern long " timezone ;
.BI "extern int " daylight ;
.fi
-.sp
+.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
-.sp
+.PP
.BR tzset ():
-_POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _POSIX_SOURCE
+_POSIX_C_SOURCE
.br
.IR tzname :
-_POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _POSIX_SOURCE
-.br
-.IR timezone :
-_SVID_SOURCE || _XOPEN_SOURCE
+_POSIX_C_SOURCE
.br
+.IR timezone ,
.IR daylight :
-_SVID_SOURCE || _XOPEN_SOURCE
+_XOPEN_SOURCE
+ || /* Glibc since 2.19: */ _DEFAULT_SOURCE
+ || /* Glibc versions <= 2.19: */ _SVID_SOURCE
.SH DESCRIPTION
The
.BR tzset ()
other time conversion functions that depend on the timezone.
In a System-V-like environment, it will also set the variables \fItimezone\fP
(seconds West of UTC) and \fIdaylight\fP (to 0 if this timezone does not
-have any daylight saving time rules, or to nonzero if there is a time during
-the year when daylight saving time applies).
+have any daylight saving time rules, or to nonzero if there is a time,
+past, present or future when daylight saving time applies).
.PP
If the
.B TZ
-variable does not appear in the environment, the \fItzname\fP
-variable is initialized with the best approximation of local wall clock
-time, as specified by the
-.BR tzfile (5)-format
-file \fIlocaltime\fP
-found in the system timezone directory (see below).
-(One also often sees
-.I /etc/localtime
-used here, a symlink to the right file in the system timezone directory.)
+variable does not appear in the environment, the system timezone is used.
+The system timezone is configured by copying, or linking, a file in the
+.BR tzfile (5)
+format to
+.IR /etc/localtime .
+A timezone database of these files may be located in the system
+timezone directory (see the \fBFILES\fP section below).
.PP
If the
.B TZ
-variable does appear in the environment but its value is empty
+variable does appear in the environment, but its value is empty,
or its value cannot be interpreted using any of the formats specified
-below, Coordinated Universal Time (UTC) is used.
+below, then Coordinated Universal Time (UTC) is used.
.PP
The value of
.B TZ
-can be one of three formats.
-The first format is used
-when there is no daylight saving time in the local timezone:
-.sp
-.RS
-.I std offset
-.RE
-.sp
-The \fIstd\fP string specifies the name of the timezone and must be
+can be one of two formats.
+The first format is a string of characters that directly represent the
+timezone to be used:
+.PP
+.in +4n
+.EX
+.IR "std offset" [ dst [ offset ][, start [ /time ], end [ /time ]]]
+.EE
+.in
+.PP
+There are no spaces in the specification.
+The \fIstd\fP string specifies an abbreviation for the timezone and must be
three or more alphabetic characters.
+When enclosed between the less-than (<) and greater-than (>) signs, the
+characters set is expanded to include the plus (+) sign, the minus (-)
+sign, and digits.
The \fIoffset\fP string immediately
follows \fIstd\fP and specifies the time value to be added to the local
time to get Coordinated Universal Time (UTC).
The \fIoffset\fP is positive
if the local timezone is west of the Prime Meridian and negative if it is
east.
-The hour must be between 0 and 24, and the minutes and seconds
-0 and 59.
-.PP
-The second format is used when there is daylight saving time:
-.sp
-.RS
-.I std offset dst [offset],start[/time],end[/time]
-.RE
-.sp
-There are no spaces in the specification.
-The initial \fIstd\fP and
-\fIoffset\fP specify the standard timezone, as described above.
+The hour must be between 0 and 24, and the minutes and seconds 00 and 59:
+.PP
+.in +4n
+.EX
+.RI [ + | - ] hh [ :mm [ :ss ]]
+.EE
+.in
+.PP
The \fIdst\fP string and \fIoffset\fP specify the name and offset for the
corresponding daylight saving timezone.
If the offset is omitted,
-it default to one hour ahead of standard time.
+it defaults to one hour ahead of standard time.
.PP
The \fIstart\fP field specifies when daylight saving time goes into
effect and the \fIend\fP field specifies when the change is made back to
The \fItime\fP fields specify when, in the local time currently in effect,
the change to the other time occurs.
If omitted, the default is 02:00:00.
-
+.PP
Here is an example for New Zealand,
where the standard time (NZST) is 12 hours ahead of UTC,
and daylight saving time (NZDT), 13 hours ahead of UTC,
runs from the first Sunday in October to the third Sunday in March,
and the changeovers happen at the default time of 02:00:00:
-.nf
-
- TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
-.fi
.PP
-The third format specifies that the timezone information should be read
+.in +4n
+.EX
+TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
+.EE
+.in
+.PP
+The second format specifies that the timezone information should be read
from a file:
-.sp
-.RS
+.PP
+.in +4n
+.EX
:[filespec]
-.RE
-.sp
-If the file specification \fIfilespec\fP is omitted, the timezone
-information is read from the file
-.I localtime
-in the system timezone directory, which nowadays usually is
-.IR /usr/share/zoneinfo .
-This file is in
-.BR tzfile (5)
-format.
+.EE
+.in
+.PP
+If the file specification \fIfilespec\fP is omitted, or its value cannot
+be interpreted, then Coordinated Universal Time (UTC) is used.
If \fIfilespec\fP is given, it specifies another
.BR tzfile (5)-format
file to read the timezone information from.
If \fIfilespec\fP does not begin with a \(aq/\(aq, the file specification is
relative to the system timezone directory.
+If the colon is omitted each
+of the above \fBTZ\fP formats will be tried.
.PP
Here's an example, once more for New Zealand:
-.nf
-
- TZ=":Pacific/Auckland"
-.fi
+.PP
+.in +4n
+.EX
+TZ=":Pacific/Auckland"
+.EE
+.in
+.SH ENVIRONMENT
+.TP
+.B TZ
+If this variable is set its value takes precedence over the system
+configured timezone.
+.TP
+.B TZDIR
+If this variable is set its value takes precedence over the system
+configured timezone database directory path.
.SH FILES
-The system timezone directory used depends on the (g)libc version.
-Libc4 and libc5 use
-.IR /usr/lib/zoneinfo ,
-and, since libc-5.4.6,
-when this doesn't work, will try
-.IR /usr/share/zoneinfo .
-Glibc2 will use the environment variable
-.BR TZDIR ,
-when that exists.
-Its default depends on how it was installed, but normally is
-.IR /usr/share/zoneinfo .
-.LP
-This timezone directory contains the files
-.nf
-localtime local timezone file
-posixrules rules for POSIX-style TZ's
-.fi
-.LP
-Often
+.TP
.I /etc/localtime
-is a symlink to the file
-.I localtime
-or to the correct timezone file in the system timezone directory.
+The system timezone file.
+.TP
+.I /usr/share/zoneinfo/
+The system timezone database directory.
+.TP
+.I /usr/share/zoneinfo/posixrules
+When a TZ string includes a dst timezone without anything following it,
+then this file is used for the start/end rules.
+It is in the
+.BR tzfile (5)
+format.
+By default, the zoneinfo Makefile hard links it to the
+.IR America/New_York " tzfile."
+.PP
+Above are the current standard file locations, but they are
+configurable when glibc is compiled.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface Attribute Value
+T{
+.BR tzset ()
+T} Thread safety MT-Safe env locale
+.TE
.SH CONFORMING TO
-SVr4, POSIX.1-2001, 4.3BSD.
+POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
.SH NOTES
-Note that the variable \fIdaylight\fP does not indicate that daylight
-saving time applies right now.
-It used to give the number of some
-algorithm (see the variable \fItz_dsttime\fP in
-.BR gettimeofday (2)).
-It has been obsolete for many years but is required by SUSv2.
-.LP
+.PP
4.3BSD had a function
.BI "char *timezone(" zone ", " dst )
that returned the
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