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  2017-09-15 "" "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)
  81 format to
  82 .IR /etc/localtime .
  83 A timezone database of these files may be located in the system
  84 timezone directory (see the \fBFILES\fP section below).
  85 .PP
  86 If the
  87 .B TZ
  88 variable does appear in the environment, but its value is empty,
  89 or its value cannot be interpreted using any of the formats specified
  90 below, then Coordinated Universal Time (UTC) is used.
  91 .PP
  92 The value of
  93 .B TZ
  94 can be one of two formats.
  95 The first format is a string of characters that directly represent the
  96 timezone to be used:
  97 .PP
  98 .in +4n
  99 .EX
 100 .IR "std offset" [ dst [ offset ][, start [ /time ], end [ /time ]]]
 101 .EE
 102 .in
 103 .PP
 104 There are no spaces in the specification.
 105 The \fIstd\fP string specifies an abbreviation for the timezone and must be
 106 three or more alphabetic characters.
 107 When enclosed between the less-than (<) and greater-than (>) signs, the
 108 characters set is expanded to include the plus (+) sign, the minus (-)
 109 sign, and digits.
 110 The \fIoffset\fP string immediately
 111 follows \fIstd\fP and specifies the time value to be added to the local
 112 time to get Coordinated Universal Time (UTC).
 113 The \fIoffset\fP is positive
 114 if the local timezone is west of the Prime Meridian and negative if it is
 115 east.
 116 The hour must be between 0 and 24, and the minutes and seconds 00 and 59:
 117 .PP
 118 .in +4n
 119 .EX
 120 .RI [ + | - ] hh [ :mm [ :ss ]]
 121 .EE
 122 .in
 123 .PP
 124 The \fIdst\fP string and \fIoffset\fP specify the name and offset for the
 125 corresponding daylight saving timezone.
 126 If the offset is omitted,
 127 it defaults to one hour ahead of standard time.
 128 .PP
 129 The \fIstart\fP field specifies when daylight saving time goes into
 130 effect and the \fIend\fP field specifies when the change is made back to
 131 standard time.
 132 These fields may have the following formats:
 133 .TP
 134 J\fIn\fP
 135 This specifies the Julian day with \fIn\fP between 1 and 365.
 136 Leap days are not counted.
 137 In this format, February 29 can't be represented;
 138 February 28 is day 59, and March 1 is always day 60.
 139 .TP
 140 .I n
 141 This specifies the zero-based Julian day with \fIn\fP between 0 and 365.
 142 February 29 is counted in leap years.
 143 .TP
 144 M\fIm\fP.\fIw\fP.\fId\fP
 145 This specifies day \fId\fP (0 <= \fId\fP <= 6) of week \fIw\fP
 146 (1 <= \fIw\fP <= 5) of month \fIm\fP (1 <= \fIm\fP <= 12).
 147 Week 1 is
 148 the first week in which day \fId\fP occurs and week 5 is the last week
 149 in which day \fId\fP occurs.
 150 Day 0 is a Sunday.
 151 .PP
 152 The \fItime\fP fields specify when, in the local time currently in effect,
 153 the change to the other time occurs.
 154 If omitted, the default is 02:00:00.
 155 .PP
 156 Here is an example for New Zealand,
 157 where the standard time (NZST) is 12 hours ahead of UTC,
 158 and daylight saving time (NZDT), 13 hours ahead of UTC,
 159 runs from the first Sunday in October to the third Sunday in March,
 160 and the changeovers happen at the default time of 02:00:00:
 161 .PP
 162 .in +4n
 163 .EX
 164 TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
 165 .EE
 166 .in
 167 .PP
 168 The second format specifies that the timezone information should be read
 169 from a file:
 170 .PP
 171 .in +4n
 172 .EX
 173 :[filespec]
 174 .EE
 175 .in
 176 .PP
 177 If the file specification \fIfilespec\fP is omitted, or its value cannot
 178 be interpreted, then Coordinated Universal Time (UTC) is used.
 179 If \fIfilespec\fP is given, it specifies another
 180 .BR tzfile (5)-format
 181 file to read the timezone information from.
 182 If \fIfilespec\fP does not begin with a \(aq/\(aq, the file specification is
 183 relative to the system timezone directory.
 184 If the colon is omitted each
 185 of the above \fBTZ\fP formats will be tried.
 186 .PP
 187 Here's an example, once more for New Zealand:
 188 .PP
 189 .in +4n
 190 .EX
 191 TZ=":Pacific/Auckland"
 192 .EE
 193 .in
 194 .SH ENVIRONMENT
 195 .TP
 196 .B TZ
 197 If this variable is set its value takes precedence over the system
 198 configured timezone.
 199 .TP
 200 .B TZDIR
 201 If this variable is set its value takes precedence over the system
 202 configured timezone database directory path.
 203 .SH FILES
 204 .TP
 205 .I /etc/localtime
 206 The system timezone file.
 207 .TP
 208 .I /usr/share/zoneinfo/
 209 The system timezone database directory.
 210 .TP
 211 .I /usr/share/zoneinfo/posixrules
 212 When a TZ string includes a dst timezone without anything following it,
 213 then this file is used for the start/end rules.
 214 It is in the
 215 .BR tzfile (5)
 216 format.
 217 By default, the zoneinfo Makefile hard links it to the
 218 .IR America/New_York " tzfile."
 219 .PP
 220 Above are the current standard file locations, but they are
 221 configurable when glibc is compiled.
 222 .SH ATTRIBUTES
 223 For an explanation of the terms used in this section, see
 224 .BR attributes (7).
 225 .TS
 226 allbox;
 227 lb lb lb
 228 l l l.
 229 Interface       Attribute       Value
 230 T{
 231 .BR tzset ()
 232 T}      Thread safety   MT-Safe env locale
 233 .TE
 234 .SH CONFORMING TO
 235 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 236 .SH NOTES
 237 .PP
 238 4.3BSD had a function
 239 .BI "char *timezone(" zone ", " dst )
 240 that returned the
 241 name of the timezone corresponding to its first argument (minutes
 242 West of UTC).
 243 If the second argument was 0, the standard name was used,
 244 otherwise the daylight saving time version.
 245 .SH SEE ALSO
 246 .BR date (1),
 247 .BR gettimeofday (2),
 248 .BR time (2),
 249 .BR ctime (3),
 250 .BR getenv (3),
 251 .BR tzfile (5)