man3/tzset.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\"
   3 .\" %%%LICENSE_START(VERBATIM)
   4 .\" Permission is granted to make and distribute verbatim copies of this
   5 .\" manual provided the copyright notice and this permission notice are
   6 .\" preserved on all copies.
   7 .\"
   8 .\" Permission is granted to copy and distribute modified versions of this
   9 .\" manual under the conditions for verbatim copying, provided that the
  10 .\" entire resulting derived work is distributed under the terms of a
  11 .\" permission notice identical to this one.
  12 .\"
  13 .\" Since the Linux kernel and libraries are constantly changing, this
  14 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  15 .\" responsibility for errors or omissions, or for damages resulting from
  16 .\" the use of the information contained herein.  The author(s) may not
  17 .\" have taken the same level of care in the production of this manual,
  18 .\" which is licensed free of charge, as they might when working
  19 .\" professionally.
  20 .\"
  21 .\" Formatted or processed versions of this manual, if unaccompanied by
  22 .\" the source, must acknowledge the copyright and authors of this work.
  23 .\" %%%LICENSE_END
  24 .\"
  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  2016-03-15 "" "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 .PP
  40 .B void tzset (void);
  41 .PP
  42 .BI "extern char *" tzname [2];
  43 .BI "extern long " timezone ;
  44 .BI "extern int " daylight ;
  45 .fi
  46 .PP
  47 .in -4n
  48 Feature Test Macro Requirements for glibc (see
  49 .BR feature_test_macros (7)):
  50 .in
  51 .PP
  52 .BR tzset ():
  53 _POSIX_C_SOURCE
  54 .br
  55 .IR tzname :
  56 _POSIX_C_SOURCE
  57 .br
  58 .IR timezone ,
  59 .IR daylight :
  60 _XOPEN_SOURCE
  61     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
  62     || /* Glibc versions <= 2.19: */ _SVID_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,
  74 past, present or future 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 .PP
 119 The \fIdst\fP string and \fIoffset\fP specify the name and offset for the
 120 corresponding daylight saving timezone.
 121 If the offset is omitted,
 122 it defaults to one hour ahead of standard time.
 123 .PP
 124 The \fIstart\fP field specifies when daylight saving time goes into
 125 effect and the \fIend\fP field specifies when the change is made back to
 126 standard time.
 127 These fields may have the following formats:
 128 .TP
 129 J\fIn\fP
 130 This specifies the Julian day with \fIn\fP between 1 and 365.
 131 Leap days are not counted.
 132 In this format, February 29 can't be represented;
 133 February 28 is day 59, and March 1 is always day 60.
 134 .TP
 135 .I n
 136 This specifies the zero-based Julian day with \fIn\fP between 0 and 365.
 137 February 29 is counted in leap years.
 138 .TP
 139 M\fIm\fP.\fIw\fP.\fId\fP
 140 This specifies day \fId\fP (0 <= \fId\fP <= 6) of week \fIw\fP
 141 (1 <= \fIw\fP <= 5) of month \fIm\fP (1 <= \fIm\fP <= 12).
 142 Week 1 is
 143 the first week in which day \fId\fP occurs and week 5 is the last week
 144 in which day \fId\fP occurs.
 145 Day 0 is a Sunday.
 146 .PP
 147 The \fItime\fP fields specify when, in the local time currently in effect,
 148 the change to the other time occurs.
 149 If omitted, the default is 02:00:00.
 150 .PP
 151 Here is an example for New Zealand,
 152 where the standard time (NZST) is 12 hours ahead of UTC,
 153 and daylight saving time (NZDT), 13 hours ahead of UTC,
 154 runs from the first Sunday in October to the third Sunday in March,
 155 and the changeovers happen at the default time of 02:00:00:
 156 .nf
 157
 158     TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
 159 .fi
 160 .PP
 161 The second format specifies that the timezone information should be read
 162 from a file:
 163 .sp
 164 .RS
 165 :[filespec]
 166 .RE
 167 .sp
 168 If the file specification \fIfilespec\fP is omitted, or its value cannot
 169 be interpreted, then Coordinated Universal Time (UTC) is used.
 170 If \fIfilespec\fP is given, it specifies another
 171 .BR tzfile (5)-format
 172 file to read the timezone information from.
 173 If \fIfilespec\fP does not begin with a \(aq/\(aq, the file specification is
 174 relative to the system timezone directory.
 175 If the colon is omitted each
 176 of the above \fBTZ\fP formats will be tried.
 177 .PP
 178 Here's an example, once more for New Zealand:
 179 .nf
 180
 181     TZ=":Pacific/Auckland"
 182 .fi
 183 .SH ENVIRONMENT
 184 .TP
 185 .B TZ
 186 If this variable is set its value takes precedence over the system
 187 configured timezone.
 188 .TP
 189 .B TZDIR
 190 If this variable is set its value takes precedence over the system
 191 configured timezone database directory path.
 192 .SH FILES
 193 .TP
 194 .I /etc/localtime
 195 The system timezone file.
 196 .TP
 197 .I /usr/share/zoneinfo/
 198 The system timezone database directory.
 199 .TP
 200 .I /usr/share/zoneinfo/posixrules
 201 When a TZ string includes a dst timezone without anything following it,
 202 then this file is used for the start/end rules.
 203 It is in the
 204 .BR tzfile "(5) format."
 205 By default, the zoneinfo Makefile hard links it to the
 206 .IR America/New_York " tzfile."
 207 .PP
 208 Above are the current standard file locations, but they are
 209 configurable when glibc is compiled.
 210 .SH ATTRIBUTES
 211 For an explanation of the terms used in this section, see
 212 .BR attributes (7).
 213 .TS
 214 allbox;
 215 lb lb lb
 216 l l l.
 217 Interface       Attribute       Value
 218 T{
 219 .BR tzset ()
 220 T}      Thread safety   MT-Safe env locale
 221 .TE
 222 .SH CONFORMING TO
 223 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 224 .SH NOTES
 225 .LP
 226 4.3BSD had a function
 227 .BI "char *timezone(" zone ", " dst )
 228 that returned the
 229 name of the timezone corresponding to its first argument (minutes
 230 West of UTC).
 231 If the second argument was 0, the standard name was used,
 232 otherwise the daylight saving time version.
 233 .SH SEE ALSO
 234 .BR date (1),
 235 .BR gettimeofday (2),
 236 .BR time (2),
 237 .BR ctime (3),
 238 .BR getenv (3),
 239 .BR tzfile (5)