.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\" have taken the same level of care in the production of this manual,
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.\" References consulted:
.\" Linux libc source code
.\" Modified 2001-11-13, aeb
.\" Modified 2004-12-01 mtk and Martin Schulze <joey@infodrom.org>
.\"
-.TH TZSET 3 2001-11-13 "" "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
+.PP
+.in -4n
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.in
+.PP
+.BR tzset ():
+_POSIX_C_SOURCE
+.br
+.IR tzname :
+_POSIX_C_SOURCE
+.br
+.IR timezone ,
+.IR daylight :
+_XOPEN_SOURCE
+ || /* Glibc since 2.19: */ _DEFAULT_SOURCE
+ || /* Glibc versions <= 2.19: */ _SVID_SOURCE
.SH DESCRIPTION
-The \fBtzset()\fP function initializes the \fItzname\fP variable from the
-TZ environment variable. This function is automatically called by the
-other time conversion functions that depend on the time zone.
-In a SysV-like environment it will also set the variables \fItimezone\fP
-(seconds West of GMT) and \fIdaylight\fP (0 if this time zone does not
-have any daylight savings time rules, nonzero if there is a time during
-the year when daylight savings time applies).
-.PP
-If the 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.)
+The
+.BR tzset ()
+function initializes the \fItzname\fP variable from the
+.B TZ
+environment variable.
+This function is automatically called by the
+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,
+past, present or future when daylight saving time applies).
+.PP
+If the
+.B TZ
+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 TZ variable does appear in the environment but its value is empty
+If the
+.B TZ
+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.
-.PP
-The value of TZ can be one of three formats. The first format is used
-when there is no daylight saving time in the local time zone:
-.sp
-.RS
-.I std offset
-.RE
-.sp
-The \fIstd\fP string specifies the name of the time zone and must be
-three or more alphabetic characters. The \fIoffset\fP string immediately
+below, then Coordinated Universal Time (UTC) is used.
+.PP
+The value of
+.B TZ
+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 time zone 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 time zone, as described above. The
-\fIdst\fP string and \fIoffset\fP specify the name and offset for the
-corresponding daylight savings time zone. If the offset is omitted,
-it defaults to one hour ahead of standard time.
-.PP
-The \fIstart\fP field specifies when daylight savings time goes into
+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 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 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
-standard time. These fields may have the following formats:
+standard time.
+These fields may have the following formats:
.TP
J\fIn\fP
-This specifies the Julian day with \fIn\fP between 1 and 365. February
-29 is never counted even in leap years.
-.TP
+This specifies the Julian day with \fIn\fP between 1 and 365.
+Leap days are not counted.
+In this format, February 29 can't be represented;
+February 28 is day 59, and March 1 is always day 60.
+.TP
.I n
-This specifies the Julian day with \fIn\fP between 1 and 365. February
-29 is counted in leap years.
-.TP
+This specifies the zero-based Julian day with \fIn\fP between 0 and 365.
+February 29 is counted in leap years.
+.TP
M\fIm\fP.\fIw\fP.\fId\fP
-This specifies day \fId\fP (0 <= \fId\fP <= 6) of week \fIw\fP
-(1 <= \fIw\fP <= 5) of month \fIm\fP (1 <= \fIm\fP <= 12). Week 1 is
-the first week in which day \fId\fP occurs and week 5 is the last week
-in which day \fId\fP occurs. Day 0 is a Sunday.
+This specifies day \fId\fP (0 <= \fId\fP <= 6) of week \fIw\fP
+(1 <= \fIw\fP <= 5) of month \fIm\fP (1 <= \fIm\fP <= 12).
+Week 1 is
+the first week in which day \fId\fP occurs and week 5 is the last week
+in which day \fId\fP occurs.
+Day 0 is a Sunday.
.PP
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.
+the change to the other time occurs.
+If omitted, the default is 02:00:00.
.PP
-The third format specifies that the time zone information should be read
+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:
+.PP
+.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 time zone
-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. If \fIfilespec\fP is given, it specifies another
+.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 time zone information from. If
-\fIfilespec\fP does not begin with a `/', the file specification is
+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:
+.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 time zone 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 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 time zone 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 time zone file in the system time zone directory.
-.SH "CONFORMING TO"
-SVID 3, POSIX, 4.3BSD
+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
+POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
.SH NOTES
-Note that the variable \fIdaylight\fP does not indicate that daylight
-savings 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
-name of the time zone corresponding to its first argument (minutes
-West of GMT). If the second argument was 0, the standard name was used,
-otherwise the daylight savings time version.
-.SH "SEE ALSO"
+name of the timezone corresponding to its first argument (minutes
+West of UTC).
+If the second argument was 0, the standard name was used,
+otherwise the daylight saving time version.
+.SH SEE ALSO
.BR date (1),
.BR gettimeofday (2),
.BR time (2),
projects / thirdparty / man-pages.git / blobdiff