man3/tzset.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  25 .\" References consulted:
  26 .\"     Linux libc source code
  27 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  28 .\"     386BSD man pages
  29 .\" Modified Sun Jul 25 11:01:58 1993 by Rik Faith (faith@cs.unc.edu)
  30 .\" Modified 2001-11-13, aeb
  31 .\" Modified 2004-12-01 mtk and Martin Schulze <joey@infodrom.org>
  32 .\"
  33 .TH TZSET 3  2015-08-08 "" "Linux Programmer's Manual"
  34 .SH NAME
  35 tzset, tzname, timezone, daylight \- initialize time conversion information
  36 .SH SYNOPSIS
  37 .nf
  38 .B #include <time.h>
  39 .sp
  40 .B void tzset (void);
  41 .sp
  42 .BI "extern char *" tzname [2];
  43 .BI "extern long " timezone ;
  44 .BI "extern int " daylight ;
  45 .fi
  46 .sp
  47 .in -4n
  48 Feature Test Macro Requirements for glibc (see
  49 .BR feature_test_macros (7)):
  50 .in
  51 .sp
  52 .BR tzset ():
  53 _POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _POSIX_SOURCE
  54 .br
  55 .IR tzname :
  56 _POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _POSIX_SOURCE
  57 .br
  58 .IR timezone :
  59 _SVID_SOURCE || _XOPEN_SOURCE
  60 .br
  61 .IR daylight :
  62 _SVID_SOURCE || _XOPEN_SOURCE
  63 .SH DESCRIPTION
  64 The
  65 .BR tzset ()
  66 function initializes the \fItzname\fP variable from the
  67 .B TZ
  68 environment variable.
  69 This function is automatically called by the
  70 other time conversion functions that depend on the timezone.
  71 In a System-V-like environment, it will also set the variables \fItimezone\fP
  72 (seconds West of UTC) and \fIdaylight\fP (to 0 if this timezone does not
  73 have any daylight saving time rules, or to nonzero if there is a time during
  74 the year when daylight saving time applies).
  75 .PP
  76 If the
  77 .B TZ
  78 variable does not appear in the environment, the system timezone is used.
  79 The system timezone is configured by copying, or linking, a file in the
  80 .BR tzfile "(5) format to"
  81 .IR /etc/localtime .
  82 A timezone database of these files may be located in the system
  83 timezone directory (see the \fBFILES\fP section below).
  84 .PP
  85 If the
  86 .B TZ
  87 variable does appear in the environment, but its value is empty,
  88 or its value cannot be interpreted using any of the formats specified
  89 below, then Coordinated Universal Time (UTC) is used.
  90 .PP
  91 The value of
  92 .B TZ
  93 can be one of two formats.
  94 The first format is a string of characters that directly represent the
  95 timezone to be used:
  96 .sp
  97 .RS
  98 .IR "std offset" [ dst [ offset ][, start [ /time ], end [ /time ]]]
  99 .RE
 100 .sp
 101 There are no spaces in the specification.
 102 The \fIstd\fP string specifies an abbreviation for the timezone and must be
 103 three or more alphabetic characters.
 104 When enclosed between the less-than (<) and greater-than (>) signs, the
 105 characters set is expanded to include the plus (+) sign, the minus (-)
 106 sign, and digits.
 107 The \fIoffset\fP string immediately
 108 follows \fIstd\fP and specifies the time value to be added to the local
 109 time to get Coordinated Universal Time (UTC).
 110 The \fIoffset\fP is positive
 111 if the local timezone is west of the Prime Meridian and negative if it is
 112 east.
 113 The hour must be between 0 and 24, and the minutes and seconds 00 and 59:
 114 .sp
 115 .RS
 116 .RI [ + | - ] hh [ :mm [ :ss ]]
 117 .RE
 118 .sp
 119 .PP
 120 The \fIdst\fP string and \fIoffset\fP specify the name and offset for the
 121 corresponding daylight saving timezone.
 122 If the offset is omitted,
 123 it defaults to one hour ahead of standard time.
 124 .PP
 125 The \fIstart\fP field specifies when daylight saving time goes into
 126 effect and the \fIend\fP field specifies when the change is made back to
 127 standard time.
 128 These fields may have the following formats:
 129 .TP
 130 J\fIn\fP
 131 This specifies the Julian day with \fIn\fP between 1 and 365.
 132 Leap days are not counted.
 133 In this format, February 29 can't be represented;
 134 February 28 is day 59, and March 1 is always day 60.
 135 .TP
 136 .I n
 137 This specifies the zero-based Julian day with \fIn\fP between 0 and 365.
 138 February 29 is counted in leap years.
 139 .TP
 140 M\fIm\fP.\fIw\fP.\fId\fP
 141 This specifies day \fId\fP (0 <= \fId\fP <= 6) of week \fIw\fP
 142 (1 <= \fIw\fP <= 5) of month \fIm\fP (1 <= \fIm\fP <= 12).
 143 Week 1 is
 144 the first week in which day \fId\fP occurs and week 5 is the last week
 145 in which day \fId\fP occurs.
 146 Day 0 is a Sunday.
 147 .PP
 148 The \fItime\fP fields specify when, in the local time currently in effect,
 149 the change to the other time occurs.
 150 If omitted, the default is 02:00:00.
 151
 152 Here is an example for New Zealand,
 153 where the standard time (NZST) is 12 hours ahead of UTC,
 154 and daylight saving time (NZDT), 13 hours ahead of UTC,
 155 runs from the first Sunday in October to the third Sunday in March,
 156 and the changeovers happen at the default time of 02:00:00:
 157 .nf
 158
 159     TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
 160 .fi
 161 .PP
 162 The second format specifies that the timezone information should be read
 163 from a file:
 164 .sp
 165 .RS
 166 :[filespec]
 167 .RE
 168 .sp
 169 If the file specification \fIfilespec\fP is omitted, or its value cannot
 170 be interpreted, then Coordinated Universal Time (UTC) is used.
 171 If \fIfilespec\fP is given, it specifies another
 172 .BR tzfile (5)-format
 173 file to read the timezone information from.
 174 If \fIfilespec\fP does not begin with a \(aq/\(aq, the file specification is
 175 relative to the system timezone directory.
 176 If the colon is omitted each
 177 of the above \fBTZ\fP formats will be tried.
 178 .PP
 179 Here's an example, once more for New Zealand:
 180 .nf
 181
 182     TZ=":Pacific/Auckland"
 183 .fi
 184 .SH ENVIRONMENT
 185 .TP
 186 .B TZ
 187 If this variable is set its value takes precedence over the system
 188 configured timezone.
 189 .TP
 190 .B TZDIR
 191 If this variable is set its value takes precedence over the system
 192 configured timezone database directory path.
 193 .SH FILES
 194 .TP
 195 .I /etc/localtime
 196 The system timezone file.
 197 .TP
 198 .I /usr/share/zoneinfo/
 199 The system timezone database directory.
 200 .TP
 201 .I /usr/share/zoneinfo/posixrules
 202 When a TZ string includes a dst timezone without anything following it,
 203 then this file is used for the start/end rules.
 204 It is in the
 205 .BR tzfile "(5) format."
 206 By default, the zoneinfo Makefile hard links it to the
 207 .IR America/New_York " tzfile."
 208 .PP
 209 Above are the current standard file locations, but they are
 210 configurable when glibc is compiled.
 211 .SH ATTRIBUTES
 212 For an explanation of the terms used in this section, see
 213 .BR attributes (7).
 214 .TS
 215 allbox;
 216 lb lb lb
 217 l l l.
 218 Interface       Attribute       Value
 219 T{
 220 .BR tzset ()
 221 T}      Thread safety   MT-Safe env locale
 222 .TE
 223 .SH CONFORMING TO
 224 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 225 .SH NOTES
 226 Note that the variable \fIdaylight\fP does not indicate that daylight
 227 saving time applies right now.
 228 It used to give the number of some
 229 algorithm (see the variable \fItz_dsttime\fP in
 230 .BR gettimeofday (2)).
 231 It has been obsolete for many years but is required by SUSv2.
 232 .LP
 233 4.3BSD had a function
 234 .BI "char *timezone(" zone ", " dst )
 235 that returned the
 236 name of the timezone corresponding to its first argument (minutes
 237 West of UTC).
 238 If the second argument was 0, the standard name was used,
 239 otherwise the daylight saving time version.
 240 .SH SEE ALSO
 241 .BR date (1),
 242 .BR gettimeofday (2),
 243 .BR time (2),
 244 .BR ctime (3),
 245 .BR getenv (3),
 246 .BR tzfile (5)