.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sun Jul 25 11:01:58 1993 by Rik Faith (faith@cs.unc.edu)
.TH TZSET 3  "July 2, 1993" "BSD" "Linux Programmer's Manual"
.SH NAME
tzset \- initialize time conversion information
.SH SYNOPSIS
.nf
.B #include <time.h>
.sp
.B void tzset (void);
.sp
.BI "extern char *" tzname [2]
.fi
.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.
.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.)
.PP
If the TZ variable does appear in the environment but its value is NULL
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
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
effect and the \fIend\fP field specifies when the change is made back to
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 
.I n
This specifies the Julian day with \fIn\fP between 1 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.
.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.
.PP
The third format specifies that the time zone information should be read
from a file:
.sp
.RS
:[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
.BR tzfile (5)-format
file to read the time zone information from.  If 
\fIfilespec\fP does not begin with a `/', the file specification is 
relative to the system timezone directory.
.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
.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, BSD 4.3
.SH "SEE ALSO"
.BR date (1),
.BR gettimeofday (2),
.BR time (2),
.BR ctime (3),
.BR getenv (3),
.BR tzfile (5)