man5/tzfile.5

   1 .\" %%%LICENSE_START(PUBLIC_DOMAIN)
   2 .\" This file is in the public domain, so clarified as of
   3 .\" 1996-06-05 by Arthur David Olson <arthur_david_olson@nih.gov>.
   4 .\" %%%LICENSE_END
   5 .\"
   6 .TH TZFILE 5 2017-08-04 "" "Linux Programmer's Manual"
   7 .SH NAME
   8 tzfile \- timezone information
   9 .SH DESCRIPTION
  10 .ie '\(lq'' .ds lq \&"\"
  11 .el .ds lq \(lq\"
  12 .ie '\(rq'' .ds rq \&"\"
  13 .el .ds rq \(rq\"
  14 .de q
  15 \\$3\*(lq\\$1\*(rq\\$2
  16 ..
  17 The timezone information files used by
  18 .BR tzset (3)
  19 are typically found under a directory with a name like
  20 .IR /usr/share/zoneinfo .
  21 These files begin with a 44-byte header containing the following fields:
  22 .IP * 2
  23 The magic four-byte ASCII sequence
  24 .q "TZif"
  25 identifies the file as a timezone information file.
  26 .IP *
  27 A byte identifying the version of the file's format
  28 (as of 2017, either an ASCII NUL, or
  29 .q "2",
  30 or
  31 .q "3" ).
  32 .IP *
  33 Fifteen bytes containing zeros reserved for future use.
  34 .IP *
  35 Six four-byte integer values
  36 written in a standard byte order
  37 (the high-order byte of the value is written first).
  38 These values are,
  39 in order:
  40 .RS
  41 .TP
  42 .I tzh_ttisgmtcnt
  43 The number of UT/local indicators stored in the file.
  44 .TP
  45 .I tzh_ttisstdcnt
  46 The number of standard/wall indicators stored in the file.
  47 .TP
  48 .I tzh_leapcnt
  49 The number of leap seconds for which data entries are stored in the file.
  50 .TP
  51 .I tzh_timecnt
  52 The number of transition times for which data entries are stored
  53 in the file.
  54 .TP
  55 .I tzh_typecnt
  56 The number of local time types for which data entries are stored
  57 in the file (must not be zero).
  58 .TP
  59 .I tzh_charcnt
  60 The number of bytes of time zone abbreviation strings
  61 stored in the file.
  62 .RE
  63 .PP
  64 The above header is followed by the following fields, whose lengths
  65 depend on the contents of the header:
  66 .IP * 2
  67 .I tzh_timecnt
  68 four-byte signed integer values sorted in ascending order.
  69 These values are written in standard byte order.
  70 Each is used as a transition time (as returned by
  71 .BR time (2))
  72 at which the rules for computing local time change.
  73 .IP *
  74 .I tzh_timecnt
  75 one-byte unsigned integer values;
  76 each one but the last tells which of the different types of local time types
  77 described in the file is associated with the time period
  78 starting with the same-indexed transition time
  79 and continuing up to but not including the next transition time.
  80 (The last time type is present only for consistency checking with the
  81 POSIX-style TZ string described below.)
  82 These values serve as indices into the next field.
  83 .IP *
  84 .I tzh_typecnt
  85 .I ttinfo
  86 entries, each defined as follows:
  87 .in +.5i
  88 .sp
  89 .nf
  90 .ta .5i +\w'unsigned char\0\0'u
  91 struct ttinfo {
  92         int32_t tt_gmtoff;
  93         unsigned char   tt_isdst;
  94         unsigned char   tt_abbrind;
  95 };
  96 .in -.5i
  97 .fi
  98 .sp
  99 Each structure is written as a four-byte signed integer value for
 100 .IR tt_gmtoff ,
 101 in a standard byte order, followed by a one-byte value for
 102 .I tt_isdst
 103 and a one-byte value for
 104 .IR tt_abbrind .
 105 In each structure,
 106 .I tt_gmtoff
 107 gives the number of seconds to be added to UT,
 108 .I tt_isdst
 109 tells whether
 110 .I tm_isdst
 111 should be set by
 112 .BR localtime (3)
 113 and
 114 .I tt_abbrind
 115 serves as an index into the array of time zone abbreviation bytes
 116 that follow the
 117 .I ttinfo
 118 structure(s) in the file.
 119 .IP *
 120 .I tzh_leapcnt
 121 pairs of four-byte values, written in standard byte order;
 122 the first value of each pair gives the nonnegative time
 123 (as returned by
 124 .BR time (2))
 125 at which a leap second occurs;
 126 the second gives the
 127 .I total
 128 number of leap seconds to be applied during the time period
 129 starting at the given time.
 130 The pairs of values are sorted in ascending order by time.
 131 Each transition is for one leap second, either positive or negative;
 132 transitions always separated by at least 28 days minus 1 second.
 133 .IP *
 134 .I tzh_ttisstdcnt
 135 standard/wall indicators, each stored as a one-byte value;
 136 they tell whether the transition times associated with local time types
 137 were specified as standard time or wall clock time,
 138 and are used when a timezone file is used in handling POSIX-style
 139 timezone environment variables.
 140 .IP *
 141 .I tzh_ttisgmtcnt
 142 UT/local indicators, each stored as a one-byte value;
 143 they tell whether the transition times associated with local time types
 144 were specified as UT or local time,
 145 and are used when a timezone file is used in handling POSIX-style
 146 timezone environment variables.
 147 .PP
 148 The
 149 .BR localtime (3)
 150 function
 151 uses the first standard-time
 152 .I ttinfo
 153 structure in the file
 154 (or simply the first
 155 .I ttinfo
 156 structure in the absence of a standard-time structure)
 157 if either
 158 .I tzh_timecnt
 159 is zero or the time argument is less than the first transition time recorded
 160 in the file.
 161 .SS Version 2 format
 162 For version-2-format timezone files,
 163 the above header and data are followed by a second header and data,
 164 identical in format except that
 165 eight bytes are used for each transition time or leap second time.
 166 (Leap second counts remain four bytes.)
 167 After the second header and data comes a newline-enclosed,
 168 POSIX-TZ-environment-variable-style string for use in handling instants
 169 after the last transition time stored in the file
 170 or for all instants if the file has no transitions.
 171 The POSIX-style TZ string is empty (i.e., nothing between the newlines)
 172 if there is no POSIX representation for such instants.
 173 If nonempty, the POSIX-style TZ string must agree with the local time
 174 type after the last transition time if present in the eight-byte data;
 175 for example, given the string
 176 .q "WET0WEST,M3.5.0,M10.5.0/3"
 177 then if a last transition time is in July, the transition's local time
 178 type must specify a daylight-saving time abbreviated
 179 .q "WEST"
 180 that is one hour east of UT.
 181 Also, if there is at least one transition, time type 0 is associated
 182 with the time period from the indefinite past up to but not including
 183 the earliest transition time.
 184 .SS Version 3 format
 185 For version-3-format timezone files, the POSIX-TZ-style string may
 186 use two minor extensions to the POSIX TZ format, as described in
 187 .BR newtzset (3).
 188 First, the hours part of its transition times may be signed and range from
 189 \-167 through 167 instead of the POSIX-required unsigned values
 190 from 0 through 24.
 191 Second, DST is in effect all year if it starts
 192 January 1 at 00:00 and ends December 31 at 24:00 plus the difference
 193 between daylight saving and standard time.
 194 .PP
 195 Future changes to the format may append more data.
 196 .SH SEE ALSO
 197 .BR time (2),
 198 .BR localtime (3),
 199 .BR tzset (3),
 200 .BR tzselect (8),
 201 .BR zdump (8),
 202 .BR zic (8)
 203 .\" This file is in the public domain, so clarified as of
 204 .\" 1996-06-05 by Arthur David Olson.