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+.TH NEWTZSET 3
+.SH NAME
+tzset \- initialize time conversion information
+.SH SYNOPSIS
+.nf
+.B void tzset()
+.PP
+.B cc ... -lz
+.fi
+.SH DESCRIPTION
+.I Tzset
+uses the value of the environment variable
+.B TZ
+to set time conversion information used by
+.IR localtime .
+If
+.B TZ
+does not appear in the environment,
+the best available approximation to local wall clock time, as specified
+by the
+.IR tzfile (5)-format
+file
+.B localtime
+in the system time conversion information directory, is used by
+.IR localtime .
+If
+.B TZ
+appears in the environment but its value is a null string,
+Coordinated Universal Time (UTC) is used (without leap second
+correction).  If
+.B TZ
+appears in the environment and its value is not a null string:
+.IP
+if the value begins with a colon, it is used as a pathname of a file
+from which to read the time conversion information;
+.IP
+if the 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,
+and, if that file cannot be read, is used directly as a specification of
+the time conversion information.
+.PP
+When
+.B TZ
+is used as a pathname, if it begins with a slash,
+it is used as an absolute pathname; otherwise,
+it is used as a pathname relative to a system time conversion information
+directory.
+The file must be in the format specified in
+.IR tzfile (5).
+.PP
+When
+.B TZ
+is used directly as a specification of the time conversion information,
+it must have the following syntax (spaces inserted for clarity):
+.IP
+\fIstd\|offset\fR[\fIdst\fR[\fIoffset\fR][\fB,\fIrule\fR]]
+.PP
+Where:
+.RS
+.TP 15
+.IR std " and " dst
+Three or more bytes that are the designation for the standard
+.RI ( std )
+or summer
+.RI ( dst )
+time zone.  Only
+.I std
+is required; if
+.I 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
+.RB ( : ),
+digits, comma
+.RB ( , ),
+minus
+.RB ( \(mi ),
+plus
+.RB ( \(pl ),
+and ASCII NUL are allowed.
+.TP
+.I offset
+Indicates the value one must add to the local time to arrive at
+Coordinated Universal Time.  The
+.I offset
+has the form:
+.RS
+.IP
+\fIhh\fR[\fB:\fImm\fR[\fB:\fIss\fR]]
+.RE
+.IP
+The minutes
+.RI ( mm )
+and seconds
+.RI ( ss )
+are optional.  The hour
+.RI ( hh )
+is required and may be a single digit.  The
+.I offset
+following
+.I std
+is required.  If no
+.I offset
+follows
+.IR 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
+.RB `` \(mi '',
+the time zone shall be east of the Prime Meridian; otherwise it shall be
+west (which may be indicated by an optional preceding
+.RB `` \(pl '').
+.TP
+.I rule
+Indicates when to change to and back from summer time.  The
+.I rule
+has the form:
+.RS
+.IP
+\fIdate\fB/\fItime\fB,\fIdate\fB/\fItime\fR
+.RE
+.IP
+where the first
+.I date
+describes when the change from standard to summer time occurs and the
+second
+.I date
+describes when the change back happens.  Each
+.I time
+field describes when, in current local time, the change to the other
+time is made.
+.IP
+The format of
+.I date
+is one of the following:
+.RS
+.TP 10
+.BI J n
+The Julian day
+.I n
+.RI "(1\ \(<=" "\ n\ " "\(<=\ 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.
+.TP
+.I n
+The zero-based Julian day
+.RI "(0\ \(<=" "\ n\ " "\(<=\ 365).
+Leap days are counted, and it is possible to refer to February 29.
+.TP
+.BI M m . n . d
+The
+.IR d' th
+day
+.RI "(0\ \(<=" "\ d\ " "\(<=\ 6)
+of week
+.I n
+of month
+.I m
+of the year
+.RI "(1\ \(<=" "\ n\ " "\(<=\ 5,
+.RI "1\ \(<=" "\ m\ " "\(<=\ 12,
+where week 5 means ``the last
+.I d
+day in month
+.IR m ''
+which may occur in either the fourth or the fifth week).  Week 1 is the
+first week in which the
+.IR d' th
+day occurs.  Day zero is Sunday.
+.RE
+.IP "" 15
+The
+.I time
+has the same format as
+.I offset
+except that no leading sign
+.RB (`` \(mi ''
+or
+.RB `` \(pl '')
+is allowed.  The default, if
+.I time
+is not given, is
+.BR 02:00:00 .
+.RE
+.LP
+If no
+.I rule
+is present in
+.BR TZ ,
+the rules specified
+by the
+.IR tzfile (5)-format
+file
+.B 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
+.I offset
+values in
+.BR TZ .
+.PP
+For compatibility with System V Release 3.1, a semicolon
+.RB ( ; )
+may be used to separate the
+.I rule
+from the rest of the specification.
+.PP
+If the
+.B TZ
+environment variable does not specify a
+.IR tzfile (5)-format
+and cannot be interpreted as a direct specification,
+UTC is used.
+.SH FILES
+.ta \w'/usr/local/etc/zoneinfo/posixrules\0\0'u
+/usr/local/etc/zoneinfo	time zone information directory
+.br
+/usr/local/etc/zoneinfo/localtime	local time zone file
+.br
+/usr/local/etc/zoneinfo/posixrules	used with POSIX-style TZ's
+.br
+/usr/local/etc/zoneinfo/GMT	for UTC leap seconds
+.sp
+If
+.B /usr/local/etc/zoneinfo/GMT
+is absent,
+UTC leap seconds are loaded from
+.BR /usr/local/etc/zoneinfo/posixrules .
+.SH SEE ALSO
+getenv(3),
+newctime(3),
+time(2),
+tzfile(5)
+.\" @(#)newtzset.3	7.3