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8<h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1>
16 <h1>Theory and pragmatics of the tz code and data</h1>
9 <h3>Outline</h3>
10 <nav>
11 <ul>
17 <h3>Outline</h3>
18 <nav>
19 <ul>
12 <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code>
13 database</a></li>
14 <li><a href="#naming">Names of time zone rulesets</a></li>
20 <li><a href="#scope">Scope of the tz database</a></li>
21 <li><a href="#naming">Names of time zone rules</a></li>
15 <li><a href="#abbreviations">Time zone abbreviations</a></li>
22 <li><a href="#abbreviations">Time zone abbreviations</a></li>
16 <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code>
17 database</a></li>
23 <li><a href="#accuracy">Accuracy of the tz database</a></li>
18 <li><a href="#functions">Time and date functions</a></li>
19 <li><a href="#stability">Interface stability</a></li>
20 <li><a href="#calendar">Calendrical issues</a></li>
21 <li><a href="#planets">Time and time zones on other planets</a></li>
22 </ul>
23 </nav>
24
24 <li><a href="#functions">Time and date functions</a></li>
25 <li><a href="#stability">Interface stability</a></li>
26 <li><a href="#calendar">Calendrical issues</a></li>
27 <li><a href="#planets">Time and time zones on other planets</a></li>
28 </ul>
29 </nav>
30
25<section>
26 <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2>
31
32 <section>
33 <h2 id="scope">Scope of the tz database</h2>
27<p>
34<p>
28The <a
29href="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code>
30database</a> attempts to record the history and predicted future of
31all computer-based clocks that track civil time.
32It organizes <a href="tz-link.html">time zone and daylight saving time
33data</a> by partitioning the world into <a
34href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones">regions</a>
35whose clocks all agree about timestamps that occur after the of the <a
36href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a>
37(1970-01-01 00:00:00 <a
38href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
39title="Coordinated Universal Time">UTC</abbr></a>).
40The database labels each such region with a notable location and
41records all known clock transitions for that location.
42Although 1970 is a somewhat-arbitrary cutoff, there are significant
43challenges to moving the cutoff earlier even by a decade or two, due
44to the wide variety of local practices before computer timekeeping
45became prevalent.
35The tz database attempts to record the history and predicted future of
36all computer-based clocks that track civil time. To represent this
37data, the world is partitioned into regions whose clocks all agree
38about timestamps that occur after the somewhat-arbitrary cutoff point
39of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region,
40the database records all known clock transitions, and labels the region
41with a notable location. Although 1970 is a somewhat-arbitrary
42cutoff, there are significant challenges to moving the cutoff earlier
43even by a decade or two, due to the wide variety of local practices
44before computer timekeeping became prevalent.
46</p>
47
48<p>
49Clock transitions before 1970 are recorded for each such location,
50because most systems support timestamps before 1970 and could
51misbehave if data entries were omitted for pre-1970 transitions.
52However, the database is not designed for and does not suffice for
53applications requiring accurate handling of all past times everywhere,
54as it would take far too much effort and guesswork to record all
55details of pre-1970 civil timekeeping.
45</p>
46
47<p>
48Clock transitions before 1970 are recorded for each such location,
49because most systems support timestamps before 1970 and could
50misbehave if data entries were omitted for pre-1970 transitions.
51However, the database is not designed for and does not suffice for
52applications requiring accurate handling of all past times everywhere,
53as it would take far too much effort and guesswork to record all
54details of pre-1970 civil timekeeping.
56Athough some information outside the scope of the database is
57collected in a file <code>backzone</code> that is distributed along
58with the database proper, this file is less reliable and does not
59necessarily follow database guidelines.
60</p>
61
62<p>
55</p>
56
57<p>
63As described below, reference source code for using the
64<code><abbr>tz</abbr></code> database is also available.
65The <code><abbr>tz</abbr></code> code is upwards compatible with <a
66href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international
67standard for <a
68href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems.
69As of this writing, the current edition of POSIX is: <a
70href="http://pubs.opengroup.org/onlinepubs/9699919799/"> The Open
71Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2008, 2016
72Edition.
73Because the database's scope encompasses real-world changes to civil
74timekeeping, its model for describing time is more complex than the
75standard and daylight saving times supported by POSIX.
76A <code><abbr>tz</abbr></code> region corresponds to a ruleset that can
77have more than two changes per year, these changes need not merely
78flip back and forth between two alternatives, and the rules themselves
79can change at times.
80Whether and when a <code><abbr>tz</abbr></code> region changes its
81clock, and even the region's notional base offset from UTC, are variable.
82It doesn't even really make sense to talk about a region's
83"base offset", since it is not necessarily a single number.
58As described below, reference source code for using the tz database is
59also available. The tz code is upwards compatible with POSIX, an
60international standard for UNIX-like systems. As of this writing, the
61current edition of POSIX is:
62 <a href="http://pubs.opengroup.org/onlinepubs/9699919799/">
63 The Open Group Base Specifications Issue 7</a>,
64 IEEE Std 1003.1-2008, 2016 Edition.
84</p>
65</p>
66 </section>
85
67
86</section>
87
68
88<section>
89 <h2 id="naming">Names of time zone rulesets</h2>
69
70 <section>
71 <h2 id="naming">Names of time zone rules</h2>
90<p>
72<p>
91Each <code><abbr>tz</abbr></code> region has a unique name that
92corresponds to a set of time zone rules.
73Each of the database's time zone rules has a unique name.
93Inexperienced users are not expected to select these names unaided.
94Distributors should provide documentation and/or a simple selection
95interface that explains the names; for one example, see the 'tzselect'
74Inexperienced users are not expected to select these names unaided.
75Distributors should provide documentation and/or a simple selection
76interface that explains the names; for one example, see the 'tzselect'
96program in the <code><abbr>tz</abbr></code> code.
97The <a href="http://cldr.unicode.org/">Unicode Common Locale Data
98Repository</a> contains data that may be useful for other selection
99interfaces.
77program in the tz code. The
78Unicode Common Locale Data
79Repository contains data that may be useful for other
80selection interfaces.
100</p>
101
102<p>
81</p>
82
83<p>
103The naming conventions attempt to strike a balance
84The time zone rule naming conventions attempt to strike a balance
104among the following goals:
105</p>
85among the following goals:
86</p>
106
107<ul>
108 <li>
87<ul>
88 <li>
109 Uniquely identify every region where clocks have agreed since 1970.
110 This is essential for the intended use: static clocks keeping local
111 civil time.
89 Uniquely identify every region where clocks have agreed since 1970.
90 This is essential for the intended use: static clocks keeping local
91 civil time.
112 </li>
113 <li>
92 </li>
93 <li>
114 Indicate to experts where that region is.
94 Indicate to experts where that region is.
115 </li>
116 <li>
95 </li>
96 <li>
117 Be robust in the presence of political changes.
118 For example, names of countries are ordinarily not used, to avoid
119 incompatibilities when countries change their name (e.g.,
120 Zaire&rarr;Congo) or when locations change countries (e.g., Hong
121 Kong from UK colony to China).
97 Be robust in the presence of political changes. For example, names
98 of countries are ordinarily not used, to avoid incompatibilities
99 when countries change their name (e.g. Zaire&rarr;Congo) or when
100 locations change countries (e.g. Hong Kong from UK colony to
101 China).
122 </li>
123 <li>
102 </li>
103 <li>
124 Be portable to a wide variety of implementations.
104 Be portable to a wide variety of implementations.
125 </li>
126 <li>
105 </li>
106 <li>
127 Use a consistent naming conventions over the entire world.
107 Use a consistent naming conventions over the entire world.
128 </li>
129</ul>
108 </li>
109</ul>
130
131<p>
110<p>
132Names normally have the form
133<var>AREA</var><code>/</code><var>LOCATION</var>, where
134<var>AREA</var> is the name of a continent or ocean, and
135<var>LOCATION</var> is the name of a specific location within that
136region.
137North and South America share the same area, '<code>America</code>'.
138Typical names are '<code>Africa/Cairo</code>',
139'<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'.
111Names normally have the
112form <var>AREA</var><code>/</code><var>LOCATION</var>,
113where <var>AREA</var> is the name of a continent or ocean,
114and <var>LOCATION</var> is the name of a specific
115location within that region. North and South America share the same
116area, '<code>America</code>'. Typical names are
117'<code>Africa/Cairo</code>', '<code>America/New_York</code>', and
118'Pacific/Honolulu'.
140</p>
141
142<p>
119</p>
120
121<p>
143Here are the general guidelines used for
144choosing <code><abbr>tz</abbr></code> region names,
122Here are the general rules used for choosing location names,
145in decreasing order of importance:
146</p>
123in decreasing order of importance:
124</p>
147
148<ul>
149 <li>
125<ul>
126 <li>
150 Use only valid POSIX file name components (i.e., the parts of
151 names other than '<code>/</code>').
152 Do not use the file name components '<code>.</code>' and
153 '<code>..</code>'.
154 Within a file name component, use only <a
155 href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters,
156 '<code>.</code>', '<code>-</code>' and '<code>_</code>'.
157 Do not use digits, as that might create an ambiguity with <a
158 href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX
159 <code>TZ</code> strings</a>.
160 A file name component must not exceed 14 characters or start with
161 '<code>-</code>'.
162 E.g., prefer '<code>Brunei</code>' to '<code>Bandar_Seri_Begawan</code>'.
163 Exceptions: see the discussion of legacy names below.
127 Use only valid POSIX file name components (i.e., the parts of
128 names other than '<code>/</code>'). Do not use the file name
129 components '<code>.</code>' and '<code>..</code>'.
130 Within a file name component,
131 use only ASCII letters, '<code>.</code>',
132 '<code>-</code>' and '<code>_</code>'. Do not use
133 digits, as that might create an ambiguity with POSIX
134 TZ strings. A file name component must not exceed 14
135 characters or start with '<code>-</code>'. E.g.,
136 prefer '<code>Brunei</code>' to
137 '<code>Bandar_Seri_Begawan</code>'. Exceptions: see
138 the discussion
139 of legacy names below.
164 </li>
165 <li>
140 </li>
141 <li>
166 A name must not be empty, or contain '<code>//</code>', or
167 start or end with '<code>/</code>'.
142 A name must not be empty, or contain '<code>//</code>', or
143 start or end with '<code>/</code>'.
168 </li>
169 <li>
144 </li>
145 <li>
170 Do not use names that differ only in case.
171 Although the reference implementation is case-sensitive, some
172 other implementations are not, and they would mishandle names
173 differing only in case.
146 Do not use names that differ only in case. Although the reference
147 implementation is case-sensitive, some other implementations
148 are not, and they would mishandle names differing only in case.
174 </li>
175 <li>
149 </li>
150 <li>
176 If one name <var>A</var> is an initial prefix of another
177 name <var>AB</var> (ignoring case), then <var>B</var> must not
178 start with '<code>/</code>', as a regular file cannot have the
179 same name as a directory in POSIX.
180 For example, '<code>America/New_York</code>' precludes
181 '<code>America/New_York/Bronx</code>'.
151 If one name <var>A</var> is an initial prefix of another
152 name <var>AB</var> (ignoring case), then <var>B</var>
153 must not start with '<code>/</code>', as a
154 regular file cannot have
155 the same name as a directory in POSIX. For example,
156 '<code>America/New_York</code>' precludes
157 '<code>America/New_York/Bronx</code>'.
182 </li>
183 <li>
158 </li>
159 <li>
184 Uninhabited regions like the North Pole and Bouvet Island
185 do not need locations, since local time is not defined there.
160 Uninhabited regions like the North Pole and Bouvet Island
161 do not need locations, since local time is not defined there.
186 </li>
187 <li>
162 </li>
163 <li>
188 There should typically be at least one name for each <a
189 href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr
190 title="International Organization for Standardization">ISO</abbr>
191 3166-1</a> officially assigned two-letter code for an inhabited
192 country or territory.
164 There should typically be at least one name for each ISO 3166-1
165 officially assigned two-letter code for an inhabited country
166 or territory.
193 </li>
194 <li>
167 </li>
168 <li>
195 If all the clocks in a region have agreed since 1970,
196 don't bother to include more than one location
197 even if subregions' clocks disagreed before 1970.
198 Otherwise these tables would become annoyingly large.
169 If all the clocks in a region have agreed since 1970,
170 don't bother to include more than one location
171 even if subregions' clocks disagreed before 1970.
172 Otherwise these tables would become annoyingly large.
199 </li>
200 <li>
173 </li>
174 <li>
201 If a name is ambiguous, use a less ambiguous alternative;
202 e.g., many cities are named San Jos�� and Georgetown, so
203 prefer '<code>Costa_Rica</code>' to '<code>San_Jose</code>' and
204 '<code>Guyana</code>' to '<code>Georgetown</code>'.
175 If a name is ambiguous, use a less ambiguous alternative;
176 e.g. many cities are named San Jos�� and Georgetown, so
177 prefer '<code>Costa_Rica</code>' to '<code>San_Jose</code>' and '<code>Guyana</code>' to '<code>Georgetown</code>'.
205 </li>
206 <li>
178 </li>
179 <li>
207 Keep locations compact.
208 Use cities or small islands, not countries or regions, so that any
209 future changes do not split individual locations into different
210 <code><abbr>tz</abbr></code> regions.
211 E.g., prefer '<code>Paris</code>' to '<code>France</code>', since
212 <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France
213 has had multiple time zones</a>.
180 Keep locations compact. Use cities or small islands, not countries
181 or regions, so that any future time zone changes do not split
182 locations into different time zones. E.g. prefer
183 '<code>Paris</code>' to '<code>France</code>', since
184 France has had multiple time zones.
214 </li>
215 <li>
185 </li>
186 <li>
216 Use mainstream English spelling, e.g., prefer '<code>Rome</code>'
217 to '<code>Roma</code>', and prefer '<code>Athens</code>' to the
218 Greek '<code>����������</code>' or the Romanized '<code>Ath��na</code>'.
219 The POSIX file name restrictions encourage this guideline.
187 Use mainstream English spelling, e.g. prefer
188 '<code>Rome</code>' to '<code>Roma</code>', and prefer
189 '<code>Athens</code>' to the Greek
190 '<code>����������</code>' or the Romanized
191 '<code>Ath��na</code>'.
192 The POSIX file name restrictions encourage this rule.
220 </li>
221 <li>
193 </li>
194 <li>
222 Use the most populous among locations in a region,
223 e.g., prefer '<code>Shanghai</code>' to
224 '<code>Beijing</code>'.
225 Among locations with similar populations, pick the best-known
226 location, e.g., prefer '<code>Rome</code>' to
227 '<code>Milan</code>'.
195 Use the most populous among locations in a zone,
196 e.g. prefer '<code>Shanghai</code>' to
197 '<code>Beijing</code>'. Among locations with
198 similar populations, pick the best-known location,
199 e.g. prefer '<code>Rome</code>' to '<code>Milan</code>'.
228 </li>
229 <li>
200 </li>
201 <li>
230 Use the singular form, e.g., prefer '<code>Canary</code>' to
231 '<code>Canaries</code>'.
202 Use the singular form, e.g. prefer '<code>Canary</code>' to '<code>Canaries</code>'.
232 </li>
233 <li>
203 </li>
204 <li>
234 Omit common suffixes like '<code>_Islands</code>' and
235 '<code>_City</code>', unless that would lead to ambiguity.
236 E.g., prefer '<code>Cayman</code>' to
237 '<code>Cayman_Islands</code>' and '<code>Guatemala</code>' to
238 '<code>Guatemala_City</code>', but prefer
239 '<code>Mexico_City</code>' to '<code>Mexico</code>'
240 because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the
241 country of Mexico has several time zones</a>.
205 Omit common suffixes like '<code>_Islands</code>' and
206 '<code>_City</code>', unless that would lead to
207 ambiguity. E.g. prefer '<code>Cayman</code>' to
208 '<code>Cayman_Islands</code>' and
209 '<code>Guatemala</code>' to
210 '<code>Guatemala_City</code>', but prefer
211 '<code>Mexico_City</code>' to '<code>Mexico</code>'
212 because the country
213 of Mexico has several time zones.
242 </li>
243 <li>
214 </li>
215 <li>
244 Use '<code>_</code>' to represent a space.
216 Use '<code>_</code>' to represent a space.
245 </li>
246 <li>
217 </li>
218 <li>
247 Omit '<code>.</code>' from abbreviations in names.
248 E.g., prefer '<code>St_Helena</code>' to '<code>St._Helena</code>'.
219 Omit '<code>.</code>' from abbreviations in names, e.g. prefer
220 '<code>St_Helena</code>' to '<code>St._Helena</code>'.
249 </li>
250 <li>
221 </li>
222 <li>
251 Do not change established names if they only marginally violate
252 the above guidelines.
253 For example, don't change the existing name '<code>Rome</code>' to
254 '<code>Milan</code>' merely because Milan's population has grown
255 to be somewhat greater than Rome's.
223 Do not change established names if they only marginally
224 violate the above rules. For example, don't change
225 the existing name '<code>Rome</code>' to
226 '<code>Milan</code>' merely because
227 Milan's population has grown to be somewhat greater
228 than Rome's.
256 </li>
257 <li>
229 </li>
230 <li>
258 If a name is changed, put its old spelling in the
259 '<code>backward</code>' file.
260 This means old spellings will continue to work.
231 If a name is changed, put its old spelling in the
232 '<code>backward</code>' file.
233 This means old spellings will continue to work.
261 </li>
262</ul>
263
264<p>
265The file '<code>zone1970.tab</code>' lists geographical locations used
234 </li>
235</ul>
236
237<p>
238The file '<code>zone1970.tab</code>' lists geographical locations used
266to name <code><abbr>tz</abbr></code> regions.
267It is intended to be an exhaustive list of names for geographic
268regions as described above; this is a subset of the names in the data.
269Although a '<code>zone1970.tab</code>' location's
270<a href="https://en.wikipedia.org/wiki/Longitude">longitude</a>
271corresponds to
272its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean
273time (<abbr>LMT</abbr>)</a> offset with one hour for every 15&deg;
274east longitude, this relationship is not exact.
239to name time
240zone rules. It is intended to be an exhaustive list of names for
241geographic regions as described above; this is a subset of the names
242in the data. Although a '<code>zone1970.tab</code>' location's longitude
243corresponds to its LMT offset with one hour for every 15 degrees east
244longitude, this relationship is not exact.
275</p>
276
277<p>
278Older versions of this package used a different naming scheme,
279and these older names are still supported.
280See the file '<code>backward</code>' for most of these older names
281(e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
282The other old-fashioned names still supported are
245</p>
246
247<p>
248Older versions of this package used a different naming scheme,
249and these older names are still supported.
250See the file '<code>backward</code>' for most of these older names
251(e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
252The other old-fashioned names still supported are
283'WET', 'CET', 'MET', and
284'<code>EET</code>' (see the file '<code>europe</code>').
253'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and '<code>EET</code>' (see the file '<code>europe</code>').
285</p>
286
287<p>
288Older versions of this package defined legacy names that are
254</p>
255
256<p>
257Older versions of this package defined legacy names that are
289incompatible with the first guideline of location names, but which are
290still supported.
291These legacy names are mostly defined in the file
292'<code>etcetera</code>'.
293Also, the file '<code>backward</code>' defines the legacy names
294'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>',
295and the file '<code>northamerica</code>' defines the legacy names
296'<code>EST5EDT</code>', '<code>CST6CDT</code>',
297'<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
258incompatible with the first rule of location names, but which are
259still supported. These legacy names are mostly defined in the file
260'<code>etcetera</code>'. Also, the file '<code>backward</code>' defines the legacy names
261'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', and the file '<code>northamerica</code>' defines the
262legacy names '<code>EST5EDT</code>', '<code>CST6CDT</code>', '<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
298</p>
299
300<p>
263</p>
264
265<p>
301Excluding 'backward' should not affect the other data.
302If '<code>backward</code>' is excluded, excluding
303'<code>etcetera</code>' should not affect the remaining data.
266Excluding '<code>backward</code>' should not affect the other data. If
267'<code>backward</code>' is excluded, excluding '<code>etcetera</code>' should not affect the
268remaining data.
304</p>
269</p>
305</section>
306
270
307<section>
308 <h2 id="abbreviations">Time zone abbreviations</h2>
271
272 </section>
273 <section>
274 <h2 id="abbreviations">Time zone abbreviations</h2>
309<p>
310When this package is installed, it generates time zone abbreviations
311like '<code>EST</code>' to be compatible with human tradition and POSIX.
275<p>
276When this package is installed, it generates time zone abbreviations
277like '<code>EST</code>' to be compatible with human tradition and POSIX.
312Here are the general guidelines used for choosing time zone abbreviations,
278Here are the general rules used for choosing time zone abbreviations,
313in decreasing order of importance:
279in decreasing order of importance:
314</p>
315
316<ul>
317 <li>
280<ul>
281 <li>
318 Use three to six characters that are ASCII alphanumerics or
319 '<code>+</code>' or '<code>-</code>'.
320 Previous editions of this database also used characters like
321 '<code> </code>' and '<code>?</code>', but these characters have a
322 special meaning to the shell and cause commands like
323 '<code><a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a>
324 `<a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>'
325 to have unexpected effects.
326 Previous editions of this guideline required upper-case letters, but the
327 Congressman who introduced
328 <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro
329 Standard Time</a> preferred "ChST", so lower-case letters are now
330 allowed.
331 Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>',
332 '<code>+</code>', and alphanumeric characters from the portable
333 character set in the current locale.
334 In practice ASCII alphanumerics and '<code>+</code>' and
335 '<code>-</code>' are safe in all locales.
282 Use three or more characters that are ASCII alphanumerics or
283 '<code>+</code>' or '<code>-</code>'.
284 Previous editions of this database also used characters like
285 '<code> </code>' and '<code>?</code>', but these
286 characters have a special meaning to
287 the shell and cause commands like
288 '<code>set `date`</code>'
289 to have unexpected effects.
290 Previous editions of this rule required upper-case letters,
291 but the Congressman who introduced Chamorro Standard Time
292 preferred "ChST", so lower-case letters are now allowed.
293 Also, POSIX from 2001 on relaxed the rule to allow
294 '<code>-</code>', '<code>+</code>',
295 and alphanumeric characters from the portable character set
296 in the current locale. In practice ASCII alphanumerics and
297 '<code>+</code>' and '<code>-</code>' are safe in all locales.
336
298
337 <p>
338 In other words, in the C locale the POSIX extended regular
339 expression <code>[-+[:alnum:]]{3,6}</code> should match the
340 abbreviation.
341 This guarantees that all abbreviations could have been specified by a
342 POSIX <code>TZ</code> string.
343 </p>
299 In other words, in the C locale the POSIX extended regular
300 expression <code>[-+[:alnum:]]{3,}</code> should match
301 the abbreviation.
302 This guarantees that all abbreviations could have been
303 specified by a POSIX TZ string.
344 </li>
345 <li>
304 </li>
305 <li>
346 Use abbreviations that are in common use among English-speakers,
347 e.g., 'EST' for Eastern Standard Time in North America.
348 We assume that applications translate them to other languages
349 as part of the normal localization process; for example,
350 a French application might translate 'EST' to 'HNE'.
351
352 <p>
353 <small>These abbreviations (for standard/daylight/etc. time) are:
354 ACST/ACDT Australian Central,
355 AST/ADT/APT/AWT/ADDT Atlantic,
356 AEST/AEDT Australian Eastern,
357 AHST/AHDT Alaska-Hawaii,
358 AKST/AKDT Alaska,
359 AWST/AWDT Australian Western,
360 BST/BDT Bering,
361 CAT/CAST Central Africa,
362 CET/CEST/CEMT Central European,
363 ChST Chamorro,
364 CST/CDT/CWT/CPT/CDDT Central [North America],
365 CST/CDT China,
366 GMT/BST/IST/BDST Greenwich,
367 EAT East Africa,
368 EST/EDT/EWT/EPT/EDDT Eastern [North America],
369 EET/EEST Eastern European,
370 GST Guam,
371 HST/HDT Hawaii,
372 HKT/HKST Hong Kong,
373 IST India,
374 IST/GMT Irish,
375 IST/IDT/IDDT Israel,
376 JST/JDT Japan,
377 KST/KDT Korea,
378 MET/MEST Middle European (a backward-compatibility alias for
379 Central European),
380 MSK/MSD Moscow,
381 MST/MDT/MWT/MPT/MDDT Mountain,
382 NST/NDT/NWT/NPT/NDDT Newfoundland,
383 NST/NDT/NWT/NPT Nome,
384 NZMT/NZST New Zealand through 1945,
385 NZST/NZDT New Zealand 1946&ndash;present,
386 PKT/PKST Pakistan,
387 PST/PDT/PWT/PPT/PDDT Pacific,
388 SAST South Africa,
389 SST Samoa,
390 WAT/WAST West Africa,
391 WET/WEST/WEMT Western European,
392 WIB Waktu Indonesia Barat,
393 WIT Waktu Indonesia Timur,
394 WITA Waktu Indonesia Tengah,
395 YST/YDT/YWT/YPT/YDDT Yukon</small>.
396 </p>
306 Use abbreviations that are in common use among English-speakers,
307 e.g. 'EST' for Eastern Standard Time in North America.
308 We assume that applications translate them to other languages
309 as part of the normal localization process; for example,
310 a French application might translate 'EST' to 'HNE'.
397 </li>
398 <li>
311 </li>
312 <li>
399 <p>
400 For times taken from a city's longitude, use the
401 traditional <var>x</var>MT notation.
402 The only abbreviation like this in current use is '<abbr>GMT</abbr>'.
403 The others are for timestamps before 1960,
404 except that Monrovia Mean Time persisted until 1972.
405 Typically, numeric abbreviations (e.g., '<code>-</code>004430' for
406 MMT) would cause trouble here, as the numeric strings would exceed
407 the POSIX length limit.
408 </p>
409
410 <p>
411 <small>These abbreviations are:
412 AMT Amsterdam, Asunci��n, Athens;
413 BMT Baghdad, Bangkok, Batavia, Bern, Bogot��, Bridgetown, Brussels,
414 Bucharest;
415 CMT Calamarca, Caracas, Chisinau, Col��n, Copenhagen, C��rdoba;
416 DMT Dublin/Dunsink;
417 EMT Easter;
418 FFMT Fort-de-France;
419 FMT Funchal;
420 GMT Greenwich;
421 HMT Havana, Helsinki, Horta, Howrah;
422 IMT Irkutsk, Istanbul;
423 JMT Jerusalem;
424 KMT Kaunas, Kiev, Kingston;
425 LMT Lima, Lisbon, local, Luanda;
426 MMT Macassar, Madras, Mal��, Managua, Minsk, Monrovia, Montevideo,
427 Moratuwa, Moscow;
428 PLMT Ph�� Li���n;
429 PMT Paramaribo, Paris, Perm, Pontianak, Prague;
430 PMMT Port Moresby;
431 QMT Quito;
432 RMT Rangoon, Riga, Rome;
433 SDMT Santo Domingo;
434 SJMT San Jos��;
435 SMT Santiago, Simferopol, Singapore, Stanley;
436 TBMT Tbilisi;
437 TMT Tallinn, Tehran;
438 WMT Warsaw</small>.
439 </p>
440
441 <p>
442 <small>A few abbreviations also follow the pattern that
443 <abbr>GMT<abbr>/<abbr>BST</abbr> established for time in the UK.
444 They are:
445 CMT/BST for Calamarca Mean Time and Bolivian Summer Time
446 1890&ndash;1932,
447 DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
448 1880&ndash;1916,
449 MMT/MST/MDST for Moscow 1880&ndash;1919, and
450 RMT/LST for Riga Mean Time and Latvian Summer time 1880&ndash;1926.
451 An extra-special case is SET for Swedish Time (<em>svensk
452 normaltid</em>) 1879&ndash;1899, 3&deg; west of the Stockholm
453 Observatory.</small>
454 </p>
313 For zones whose times are taken from a city's longitude, use the
314 traditional <var>x</var>MT notation, e.g. 'PMT' for
315 Paris Mean Time.
316 The only name like this in current use is 'GMT'.
455 </li>
456 <li>
317 </li>
318 <li>
457 Use '<abbr>LMT</abbr>' for local mean time of locations before the
458 introduction of standard time; see "<a href="#scope">Scope of the
459 <code><abbr>tz</abbr></code> database</a>".
319 Use 'LMT' for local mean time of locations before the introduction
320 of standard time; see "<a href="#scope">Scope of the
321 tz database</a>".
460 </li>
461 <li>
322 </li>
323 <li>
462 If there is no common English abbreviation, use numeric offsets like
463 <code>-</code>05 and <code>+</code>0830 that are generated
464 by <code>zic</code>'s <code>%z</code> notation.
324 If there is no common English abbreviation, use numeric offsets like
325 <code>-</code>05 and <code>+</code>0830 that are
326 generated by zic's <code>%z</code> notation.
465 </li>
466 <li>
327 </li>
328 <li>
467 Use current abbreviations for older timestamps to avoid confusion.
468 For example, in 1910 a common English abbreviation for time
469 in central Europe was 'MEZ' (short for both "Middle European
470 Zone" and for "Mitteleurop��ische Zeit" in German).
471 Nowadays 'CET' ("Central European Time") is more common in
472 English, and the database uses 'CET' even for circa-1910
473 timestamps as this is less confusing for modern users and avoids
474 the need for determining when 'CET' supplanted 'MEZ' in common
475 usage.
329 Use current abbreviations for older timestamps to avoid confusion.
330 For example, in 1910 a common English abbreviation for UT +01
331 in central Europe was 'MEZ' (short for both "Middle European
332 Zone" and for "Mitteleurop��ische Zeit" in German). Nowadays
333 'CET' ("Central European Time") is more common in English, and
334 the database uses 'CET' even for circa-1910 timestamps as this
335 is less confusing for modern users and avoids the need for
336 determining when 'CET' supplanted 'MEZ' in common usage.
476 </li>
477 <li>
337 </li>
338 <li>
478 Use a consistent style in a <code><abbr>tz</abbr></code> region's history.
479 For example, if history tends to use numeric
480 abbreviations and a particular entry could go either way, use a
481 numeric abbreviation.
339 Use a consistent style in a zone's history. For example, if a zone's
340 history tends to use numeric abbreviations and a particular
341 entry could go either way, use a numeric abbreviation.
482 </li>
342 </li>
343</ul>
344 [The remaining guidelines predate the introduction of <code>%z</code>.
345 They are problematic as they mean tz data entries invent
346 notation rather than record it. These guidelines are now
347 deprecated and the plan is to gradually move to <code>%z</code> for
348 inhabited locations and to "<code>-</code>00" for uninhabited locations.]
349<ul>
483 <li>
350 <li>
484 Use
485 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a>
486 (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for
487 locations while uninhabited.
488 The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in
489 some sense undefined; this notation is derived
490 from <a href="https://tools.ietf.org/html/rfc3339">Internet
491 <abbr title="Request For Comments">RFC 3339</a>.
351 If there is no common English abbreviation, abbreviate the English
352 translation of the usual phrase used by native speakers.
353 If this is not available or is a phrase mentioning the country
354 (e.g. "Cape Verde Time"), then:
355 <ul>
356 <li>
357 When a country is identified with a single or principal zone,
358 append 'T' to the country's ISO code, e.g. 'CVT' for
359 Cape Verde Time. For summer time append 'ST';
360 for double summer time append 'DST'; etc.
361 </li>
362 <li>
363 Otherwise, take the first three letters of an English place
364 name identifying each zone and append 'T', 'ST', etc.
365 as before; e.g. 'CHAST' for CHAtham Summer Time.
366 </li>
367 </ul>
492 </li>
368 </li>
369 <li>
370 Use UT (with time zone abbreviation '<code>-</code>00') for
371 locations while uninhabited. The leading
372 '<code>-</code>' is a flag that the time
373 zone is in some sense undefined; this notation is
374 derived from Internet RFC 3339.
375 </li>
493</ul>
376</ul>
494
495<p>
496Application writers should note that these abbreviations are ambiguous
377<p>
378Application writers should note that these abbreviations are ambiguous
497in practice: e.g., 'CST' means one thing in China and something else
498in North America, and 'IST' can refer to time in India, Ireland or
499Israel.
500To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
501'<code>-</code>0600' instead of time zone abbreviations like 'CST'.
379in practice: e.g. 'CST' has a different meaning in China than
380it does in the United States. In new applications, it's often better
381to use numeric UT offsets like '<code>-</code>0600' instead of time zone
382abbreviations like 'CST'; this avoids the ambiguity.
502</p>
383</p>
503
384 </section>
504
385
505<section>
506 <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
386
387 <section>
388 <h2 id="accuracy">Accuracy of the tz database</h2>
507<p>
389<p>
508The <code><abbr>tz</abbr></code> database is not authoritative, and it
509surely has errors.
510Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
390The tz database is not authoritative, and it surely has errors.
391Corrections are welcome and encouraged; see the file CONTRIBUTING.
511Users requiring authoritative data should consult national standards
512bodies and the references cited in the database's comments.
513</p>
514
515<p>
392Users requiring authoritative data should consult national standards
393bodies and the references cited in the database's comments.
394</p>
395
396<p>
516Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
397Errors in the tz database arise from many sources:
517</p>
398</p>
518
519<ul>
520 <li>
399<ul>
400 <li>
521 The <code><abbr>tz</abbr></code> database predicts future
522 timestamps, and current predictions
523 will be incorrect after future governments change the rules.
524 For example, if today someone schedules a meeting for 13:00 next
525 October 1, Casablanca time, and tomorrow Morocco changes its
526 daylight saving rules, software can mess up after the rule change
527 if it blithely relies on conversions made before the change.
401 The tz database predicts future timestamps, and current predictions
402 will be incorrect after future governments change the rules.
403 For example, if today someone schedules a meeting for 13:00 next
404 October 1, Casablanca time, and tomorrow Morocco changes its
405 daylight saving rules, software can mess up after the rule change
406 if it blithely relies on conversions made before the change.
528 </li>
529 <li>
407 </li>
408 <li>
530 The pre-1970 entries in this database cover only a tiny sliver of how
531 clocks actually behaved; the vast majority of the necessary
532 information was lost or never recorded.
533 Thousands more <code><abbr>tz</abbr></code> regions would be needed if
534 the <code><abbr>tz</abbr></code> database's scope were extended to
535 cover even just the known or guessed history of standard time; for
536 example, the current single entry for France would need to split
537 into dozens of entries, perhaps hundreds.
538 And in most of the world even this approach would be misleading
539 due to widespread disagreement or indifference about what times
540 should be observed.
541 In her 2015 book
542 <cite><a
543 href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The
544 Global Transformation of Time, 1870&ndash;1950</a></cite>,
545 Vanessa Ogle writes
546 "Outside of Europe and North America there was no system of time
547 zones at all, often not even a stable landscape of mean times,
548 prior to the middle decades of the twentieth century".
549 See: Timothy Shenk, <a
550href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
551 A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
409 The pre-1970 entries in this database cover only a tiny sliver of how
410 clocks actually behaved; the vast majority of the necessary
411 information was lost or never recorded. Thousands more zones would
412 be needed if the tz database's scope were extended to cover even
413 just the known or guessed history of standard time; for example,
414 the current single entry for France would need to split into dozens
415 of entries, perhaps hundreds. And in most of the world even this
416 approach would be misleading due to widespread disagreement or
417 indifference about what times should be observed. In her 2015 book
418 <cite>The Global Transformation of Time, 1870-1950</cite>, Vanessa Ogle writes
419 "Outside of Europe and North America there was no system of time
420 zones at all, often not even a stable landscape of mean times,
421 prior to the middle decades of the twentieth century". See:
422 Timothy Shenk, <a
423 href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
424 A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
552 </li>
553 <li>
425 </li>
426 <li>
554 Most of the pre-1970 data entries come from unreliable sources, often
555 astrology books that lack citations and whose compilers evidently
556 invented entries when the true facts were unknown, without
557 reporting which entries were known and which were invented.
558 These books often contradict each other or give implausible entries,
559 and on the rare occasions when they are checked they are
560 typically found to be incorrect.
427 Most of the pre-1970 data entries come from unreliable sources, often
428 astrology books that lack citations and whose compilers evidently
429 invented entries when the true facts were unknown, without
430 reporting which entries were known and which were invented.
431 These books often contradict each other or give implausible entries,
432 and on the rare occasions when they are checked they are
433 typically found to be incorrect.
561 </li>
562 <li>
434 </li>
435 <li>
563 For the UK the <code><abbr>tz</abbr></code> database relies on
564 years of first-class work done by
565 Joseph Myers and others; see
566 "<a href="https://www.polyomino.org.uk/british-time/">History of
567 legal time in Britain</a>".
568 Other countries are not done nearly as well.
436 For the UK the tz database relies on years of first-class work done by
437 Joseph Myers and others; see
438 "<a href="https://www.polyomino.org.uk/british-time/">History of
439 legal time in Britain</a>".
440 Other countries are not done nearly as well.
569 </li>
570 <li>
441 </li>
442 <li>
571 Sometimes, different people in the same city maintain clocks
572 that differ significantly.
573 Historically, railway time was used by railroad companies (which
574 did not always
575 agree with each other), church-clock time was used for birth
576 certificates, etc.
577 More recently, competing political groups might disagree about
578 clock settings. Often this is merely common practice, but
579 sometimes it is set by law.
580 For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France
581 was legally <abbr>UT</abbr> +00:09:21 outside train stations and
582 <abbr>UT</abbr> +00:04:21 inside. Other examples include
583 Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and
584 ��r��mqi to this day.
443 Sometimes, different people in the same city would maintain clocks
444 that differed significantly. Railway time was used by railroad
445 companies (which did not always agree with each other),
446 church-clock time was used for birth certificates, etc.
447 Often this was merely common practice, but sometimes it was set by law.
448 For example, from 1891 to 1911 the UT offset in France was legally
449 0:09:21 outside train stations and 0:04:21 inside.
585 </li>
586 <li>
450 </li>
451 <li>
587 Although a named location in the <code><abbr>tz</abbr></code>
588 database stands for the containing region, its pre-1970 data
589 entries are often accurate for only a small subset of that region.
590 For example, <code>Europe/London</code> stands for the United
591 Kingdom, but its pre-1847 times are valid only for locations that
592 have London's exact meridian, and its 1847 transition
593 to <abbr>GMT</abbr> is known to be valid only for the L&amp;NW and
594 the Caledonian railways.
452 Although a named location in the tz database stands for the
453 containing region, its pre-1970 data entries are often accurate for
454 only a small subset of that region. For example, <code>Europe/London</code>
455 stands for the United Kingdom, but its pre-1847 times are valid
456 only for locations that have London's exact meridian, and its 1847
457 transition to GMT is known to be valid only for the L&amp;NW and the
458 Caledonian railways.
595 </li>
596 <li>
459 </li>
460 <li>
597 The <code><abbr>tz</abbr></code> database does not record the
598 earliest time for which a <code><abbr>tz</abbr></code> region's
599 data entries are thereafter valid for every location in the region.
600 For example, <code>Europe/London</code> is valid for all locations
601 in its region after <abbr>GMT</abbr> was made the standard time,
602 but the date of standardization (1880-08-02) is not in the
603 <code><abbr>tz</abbr></code> database, other than in commentary.
604 For many <code><abbr>tz</abbr></code> regions the earliest time of
605 validity is unknown.
461 The tz database does not record the earliest time for which a zone's
462 data entries are thereafter valid for every location in the region.
463 For example, <code>Europe/London</code> is valid for all locations in its
464 region after GMT was made the standard time, but the date of
465 standardization (1880-08-02) is not in the tz database, other than
466 in commentary. For many zones the earliest time of validity is
467 unknown.
606 </li>
607 <li>
468 </li>
469 <li>
608 The <code><abbr>tz</abbr></code> database does not record a
609 region's boundaries, and in many cases the boundaries are not known.
610 For example, the <code><abbr>tz</abbr></code> region
611 <code>America/Kentucky/Louisville</code> represents a region
612 around the city of Louisville, the boundaries of which are
613 unclear.
470 The tz database does not record a region's boundaries, and in many
471 cases the boundaries are not known. For example, the zone
472 <code>America/Kentucky/Louisville</code> represents a region around
473 the city of
474 Louisville, the boundaries of which are unclear.
614 </li>
615 <li>
475 </li>
476 <li>
616 Changes that are modeled as instantaneous transitions in the
617 <code><abbr>tz</abbr></code>
618 database were often spread out over hours, days, or even decades.
477 Changes that are modeled as instantaneous transitions in the tz
478 database were often spread out over hours, days, or even decades.
619 </li>
620 <li>
479 </li>
480 <li>
621 Even if the time is specified by law, locations sometimes
622 deliberately flout the law.
481 Even if the time is specified by law, locations sometimes
482 deliberately flout the law.
623 </li>
624 <li>
483 </li>
484 <li>
625 Early timekeeping practices, even assuming perfect clocks, were
626 often not specified to the accuracy that the
627 <code><abbr>tz</abbr></code> database requires.
485 Early timekeeping practices, even assuming perfect clocks, were
486 often not specified to the accuracy that the tz database requires.
628 </li>
629 <li>
487 </li>
488 <li>
630 Sometimes historical timekeeping was specified more precisely
631 than what the <code><abbr>tz</abbr></code> code can handle.
632 For example, from 1909 to 1937 <a
633 href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
634 hreflang="nl">Netherlands clocks</a> were legally Amsterdam Mean
635 Time (estimated to be <abbr>UT</abbr>
636 +00:19:32.13), but the <code><abbr>tz</abbr></code>
637 code cannot represent the fractional second.
638 In practice these old specifications were rarely if ever
639 implemented to subsecond precision.
489 Sometimes historical timekeeping was specified more precisely
490 than what the tz database can handle. For example, from 1909 to
491 1937 Netherlands clocks were legally UT +00:19:32.13, but the tz
492 database cannot represent the fractional second.
640 </li>
641 <li>
493 </li>
494 <li>
642 Even when all the timestamp transitions recorded by the
643 <code><abbr>tz</abbr></code> database are correct, the
644 <code><abbr>tz</abbr></code> rules that generate them may not
645 faithfully reflect the historical rules.
646 For example, from 1922 until World War II the UK moved clocks
647 forward the day following the third Saturday in April unless that
648 was Easter, in which case it moved clocks forward the previous
649 Sunday.
650 Because the <code><abbr>tz</abbr></code> database has no
651 way to specify Easter, these exceptional years are entered as
652 separate <code><abbr>tz</abbr> Rule</code> lines, even though the
653 legal rules did not change.
495 Even when all the timestamp transitions recorded by the tz database
496 are correct, the tz rules that generate them may not faithfully
497 reflect the historical rules. For example, from 1922 until World
498 War II the UK moved clocks forward the day following the third
499 Saturday in April unless that was Easter, in which case it moved
500 clocks forward the previous Sunday. Because the tz database has no
501 way to specify Easter, these exceptional years are entered as
502 separate tz Rule lines, even though the legal rules did not change.
654 </li>
655 <li>
503 </li>
504 <li>
656 The <code><abbr>tz</abbr></code> database models pre-standard time
657 using the <a
658 href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
659 Gregorian calendar</a> and local mean time, but many people used
660 other calendars and other timescales.
661 For example, the Roman Empire used
662 the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
663 calendar</a>,
664 and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman
665 timekeeping</a> had twelve varying-length daytime hours with a
666 non-hour-based system at night.
505 The tz database models pre-standard time using the proleptic Gregorian
506 calendar and local mean time (LMT), but many people used other
507 calendars and other timescales. For example, the Roman Empire used
508 the Julian calendar, and had 12 varying-length daytime hours with a
509 non-hour-based system at night.
667 </li>
668 <li>
510 </li>
511 <li>
669 Early clocks were less reliable, and data entries do not represent
670 clock error.
512 Early clocks were less reliable, and data entries do not represent
513 clock error.
671 </li>
672 <li>
514 </li>
515 <li>
673 The <code><abbr>tz</abbr></code> database assumes Universal Time
674 (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not
675 standardized for older timestamps.
676 In the <code><abbr>tz</abbr></code> database commentary,
677 <abbr>UT</abbr> denotes a family of time standards that includes
678 Coordinated Universal Time (<abbr>UTC</abbr>) along with other
679 variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>,
680 with days starting at midnight.
681 Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern
682 timestamps, <abbr>UTC</abbr> was not defined until 1960, so
683 commentary uses the more-general abbreviation <abbr>UT</abbr> for
684 timestamps that might predate 1960.
685 Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly,
686 and since pre-1972 <abbr>UTC</abbr> seconds varied in length,
687 interpretation of older timestamps can be problematic when
688 subsecond accuracy is needed.
516 The tz database assumes Universal Time (UT) as an origin, even
517 though UT is not standardized for older timestamps. In the tz
518 database commentary, UT denotes a family of time standards that
519 includes Coordinated Universal Time (UTC) along with other variants
520 such as UT1 and GMT, with days starting at midnight. Although UT
521 equals UTC for modern timestamps, UTC was not defined until 1960,
522 so commentary uses the more-general abbreviation UT for timestamps
523 that might predate 1960. Since UT, UT1, etc. disagree slightly,
524 and since pre-1972 UTC seconds varied in length, interpretation of
525 older timestamps can be problematic when subsecond accuracy is
526 needed.
689 </li>
690 <li>
527 </li>
528 <li>
691 Civil time was not based on atomic time before 1972, and we don't
692 know the history of
693 <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's
694 rotation</a> accurately enough to map <a
695 href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr
696 title="International System of Units">SI</abbr></a> seconds to
697 historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a>
698 to more than about one-hour accuracy.
699 See: Stephenson FR, Morrison LV, Hohenkerk CY.
700 <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement of
701 the Earth's rotation: 720 BC to AD 2015</a>.
702 <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404.
703 Also see: Espenak F. <a
704 href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
705 in Delta T (��T)</a>.
529 Civil time was not based on atomic time before 1972, and we don't
530 know the history of earth's rotation accurately enough to map SI
531 seconds to historical solar time to more than about one-hour
532 accuracy. See: Stephenson FR, Morrison LV, Hohenkerk CY.
533 <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement
534 of the Earth's rotation: 720 BC to AD 2015</a>.
535 <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404.
536 Also see: Espenak F. <a
537 href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
538 in Delta T (��T)</a>.
706 </li>
707 <li>
539 </li>
540 <li>
708 The relationship between POSIX time (that is, <abbr>UTC</abbr> but
709 ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap
710 seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972.
711 Although the POSIX
712 clock officially stops during an inserted leap second, at least one
713 proposed standard has it jumping back a second instead; and in
714 practice POSIX clocks more typically either progress glacially during
715 a leap second, or are slightly slowed while near a leap second.
541 The relationship between POSIX time (that is, UTC but ignoring leap
542 seconds) and UTC is not agreed upon after 1972. Although the POSIX
543 clock officially stops during an inserted leap second, at least one
544 proposed standard has it jumping back a second instead; and in
545 practice POSIX clocks more typically either progress glacially during
546 a leap second, or are slightly slowed while near a leap second.
716 </li>
717 <li>
547 </li>
548 <li>
718 The <code><abbr>tz</abbr></code> database does not represent how
719 uncertain its information is.
720 Ideally it would contain information about when data entries are
721 incomplete or dicey.
722 Partial temporal knowledge is a field of active research, though,
723 and it's not clear how to apply it here.
549 The tz database does not represent how uncertain its information is.
550 Ideally it would contain information about when data entries are
551 incomplete or dicey. Partial temporal knowledge is a field of
552 active research, though, and it's not clear how to apply it here.
724 </li>
725</ul>
553 </li>
554</ul>
726
727<p>
555<p>
728In short, many, perhaps most, of the <code><abbr>tz</abbr></code>
729database's pre-1970 and future timestamps are either wrong or
730misleading.
731Any attempt to pass the
732<code><abbr>tz</abbr></code> database off as the definition of time
733should be unacceptable to anybody who cares about the facts.
734In particular, the <code><abbr>tz</abbr></code> database's
735<abbr>LMT</abbr> offsets should not be considered meaningful, and
736should not prompt creation of <code><abbr>tz</abbr></code> regions
737merely because two locations
738differ in <abbr>LMT</abbr> or transitioned to standard time at
739different dates.
556In short, many, perhaps most, of the tz database's pre-1970 and future
557timestamps are either wrong or misleading. Any attempt to pass the
558tz database off as the definition of time should be unacceptable to
559anybody who cares about the facts. In particular, the tz database's
560LMT offsets should not be considered meaningful, and should not prompt
561creation of zones merely because two locations differ in LMT or
562transitioned to standard time at different dates.
740</p>
563</p>
741
564 </section>
742
565
743<section>
744 <h2 id="functions">Time and date functions</h2>
566
567 <section>
568 <h2 id="functions">Time and date functions</h2>
745<p>
569<p>
746The <code><abbr>tz</abbr></code> code contains time and date functions
747that are upwards compatible with those of POSIX.
748Code compatible with this package is already
749<a href="tz-link.html#tzdb">part of many platforms</a>, where the
750primary use of this package is to update obsolete time-related files.
751To do this, you may need to compile the time zone compiler
752'<code>zic</code>' supplied with this package instead of using the
753system '<code>zic</code>', since the format of <code>zic</code>'s
754input is occasionally extended, and a platform may still be shipping
755an older <code>zic</code>.
570The tz code contains time and date functions that are upwards
571compatible with those of POSIX.
756</p>
757
572</p>
573
758<h3 id="POSIX">POSIX properties and limitations</h3>
574<p>
575POSIX has the following properties and limitations.
576</p>
759<ul>
760 <li>
761 <p>
577<ul>
578 <li>
579 <p>
762 In POSIX, time display in a process is controlled by the
763 environment variable <code>TZ</code>.
764 Unfortunately, the POSIX
765 <code>TZ</code> string takes a form that is hard to describe and
766 is error-prone in practice.
767 Also, POSIX <code>TZ</code> strings can't deal with daylight
768 saving time rules not based on the Gregorian calendar (as in
769 Iran), or with situations where more than two time zone
770 abbreviations or <abbr>UT</abbr> offsets are used in an area.
580 In POSIX, time display in a process is controlled by the
581 environment variable TZ. Unfortunately, the POSIX TZ string takes
582 a form that is hard to describe and is error-prone in practice.
583 Also, POSIX TZ strings can't deal with other (for example, Israeli)
584 daylight saving time rules, or situations where more than two
585 time zone abbreviations are used in an area.
771 </p>
586 </p>
772
773 <p>
587 <p>
774 The POSIX <code>TZ</code> string takes the following form:
588 The POSIX TZ string takes the following form:
775 </p>
589 </p>
776
777 <p>
590 <p>
778 stdoffset[dst[offset][,date[/time],date[/time]]]
591 <var>stdoffset</var>[<var>dst</var>[<var>offset</var>][<code>,</code><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]]]
779 </p>
592 </p>
780
781 <p>
593 <p>
782 where:
783 </p>
784
594 where:
785 <dl>
786 <dt><var>std</var> and <var>dst</var></dt><dd>
595 <dl>
596 <dt><var>std</var> and <var>dst</var></dt><dd>
787 are 3 or more characters specifying the standard
788 and daylight saving time (<abbr>DST</abbr>) zone names.
789 Starting with POSIX.1-2001, <var>std</var> and <var>dst</var>
790 may also be in a quoted form like '<code>&lt;+09&gt;</code>';
791 this allows "<code>+</code>" and "<code>-</code>" in the names.
597 are 3 or more characters specifying the standard
598 and daylight saving time (DST) zone names.
599 Starting with POSIX.1-2001, <var>std</var>
600 and <var>dst</var> may also be
601 in a quoted form like '<code>&lt;UTC+10&gt;</code>'; this allows
602 "<code>+</code>" and "<code>-</code>" in the names.
792 </dd>
793 <dt><var>offset</var></dt><dd>
603 </dd>
604 <dt><var>offset</var></dt><dd>
794 is of the form
795 '[±]hh:[mm[:ss]]'
796 and specifies the offset west of <abbr>UT</abbr>.
797 '<var>hh</var>' may be a single digit;
798 0&le;<var>hh</var>&le;24.
799 The default <abbr>DST</abbr> offset is one hour ahead of
800 standard time.
605 is of the form
606 '<code>[&plusmn;]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>'
607 and specifies the offset west of UT. '<var>hh</var>'
608 may be a single digit; 0&le;<var>hh</var>&le;24.
609 The default DST offset is one hour ahead of standard time.
801 </dd>
802 <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
610 </dd>
611 <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
803 specifies the beginning and end of <abbr>DST</abbr>.
804 If this is absent, the system supplies its own ruleset
805 for <abbr>DST</abbr>, and its rules can differ from year to year;
806 typically <abbr>US</abbr> <abbr>DST</abbr> rules are used.
612 specifies the beginning and end of DST. If this is absent,
613 the system supplies its own rules for DST, and these can
614 differ from year to year; typically US DST rules are used.
807 </dd>
808 <dt><var>time</var></dt><dd>
615 </dd>
616 <dt><var>time</var></dt><dd>
809 takes the form
810 'hh:[mm[:ss]]'
811 and defaults to 02:00.
812 This is the same format as the offset, except that a
813 leading '+' or '-' is not allowed.
617 takes the form
618 '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]'
619 and defaults to 02:00.
620 This is the same format as the offset, except that a
621 leading '<code>+</code>' or '<code>-</code>' is not allowed.
814 </dd>
815 <dt><var>date</var></dt><dd>
622 </dd>
623 <dt><var>date</var></dt><dd>
816 takes one of the following forms:
624 takes one of the following forms:
817 <dl>
818 <dt>J<var>n</var> (1&le;<var>n</var>&le;365)</dt><dd>
625 <dl>
626 <dt>J<var>n</var> (1&le;<var>n</var>&le;365)</dt><dd>
819 origin-1 day number not counting February 29
820 </dd>
627 origin-1 day number not counting February 29
628 </dd>
821 <dt><var>n</var> (0&le;<var>n</var>&le;365)</dt><dd>
629 <dt><var>n</var> (0&le;<var>n</var>&le;365)</dt><dd>
822 origin-0 day number counting February 29 if present
630 origin-0 day number counting February 29 if present
631 </dd>
632 <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> (0[Sunday]&le;<var>d</var>&le;6[Saturday], 1&le;<var>n</var>&le;5, 1&le;<var>m</var>&le;12)</dt><dd>
633 for the <var>d</var>th day of
634 week <var>n</var> of month <var>m</var> of the
635 year, where week 1 is the first week in which
636 day <var>d</var> appears, and '<code>5</code>'
637 stands for the last week in which
638 day <var>d</var> appears
639 (which may be either the 4th or 5th week).
640 Typically, this is the only useful form;
641 the <var>n</var>
642 and <code>J</code><var>n</var> forms are
643 rarely used.
823 </dd>
644 </dd>
824 <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var>
825 (0[Sunday]&le;<var>d</var>&le;6[Saturday], 1&le;<var>n</var>&le;5,
826 1&le;<var>m</var>&le;12)</dt><dd>
827 for the <var>d</var>th day of week <var>n</var> of
828 month <var>m</var> of the year, where week 1 is the first
829 week in which day <var>d</var> appears, and
830 '<code>5</code>' stands for the last week in which
831 day <var>d</var> appears (which may be either the 4th or
832 5th week).
833 Typically, this is the only useful form; the <var>n</var>
834 and <code>J</code><var>n</var> forms are rarely used.
835 </dd>
836 </dl>
837 </dd>
838 </dl>
645</dl>
646</dd>
647</dl>
648 Here is an example POSIX TZ string for New Zealand after 2007.
649 It says that standard time (NZST) is 12 hours ahead of UTC,
650 and that daylight saving time (NZDT) is observed from September's
651 last Sunday at 02:00 until April's first Sunday at 03:00:
839
652
840 <p>
841 Here is an example POSIX <code>TZ</code> string for New
842 Zealand after 2007.
843 It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead
844 of <abbr>UT</abbr>, and that daylight saving time
845 (<abbr>NZDT</abbr>) is observed from September's last Sunday at
846 02:00 until April's first Sunday at 03:00:
847 </p>
653 <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
848
654
849 <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
655 This POSIX TZ string is hard to remember, and mishandles some
656 timestamps before 2008. With this package you can use this
657 instead:
850
658
851 <p>
852 This POSIX <code>TZ</code> string is hard to remember, and
853 mishandles some timestamps before 2008.
854 With this package you can use this instead:
855 </p>
856
857 <pre><code>TZ='Pacific/Auckland'</code></pre>
659 <pre><code>TZ='Pacific/Auckland'</code></pre>
858 </li>
859 <li>
660 </li>
661 <li>
860 POSIX does not define the exact meaning of <code>TZ</code> values like
861 "<code>EST5EDT</code>".
862 Typically the current <abbr>US</abbr> <abbr>DST</abbr> rules
863 are used to interpret such values, but this means that the
864 <abbr>US</abbr> <abbr>DST</abbr> rules are compiled into each
865 program that does time conversion.
866 This means that when
867 <abbr>US</abbr> time conversion rules change (as in the United
868 States in 1987), all programs that do time conversion must be
869 recompiled to ensure proper results.
662 POSIX does not define the exact meaning of TZ values like
663 "<code>EST5EDT</code>".
664 Typically the current US DST rules are used to interpret such values,
665 but this means that the US DST rules are compiled into each program
666 that does time conversion. This means that when US time conversion
667 rules change (as in the United States in 1987), all programs that
668 do time conversion must be recompiled to ensure proper results.
870 </li>
871 <li>
669 </li>
670 <li>
872 The <code>TZ</code> environment variable is process-global, which
873 makes it hard to write efficient, thread-safe applications that
874 need access to multiple time zone rulesets.
671 The TZ environment variable is process-global, which makes it hard
672 to write efficient, thread-safe applications that need access
673 to multiple time zones.
875 </li>
876 <li>
674 </li>
675 <li>
877 In POSIX, there's no tamper-proof way for a process to learn the
878 system's best idea of local wall clock.
879 (This is important for applications that an administrator wants
880 used only at certain times &ndash; without regard to whether the
881 user has fiddled the
882 <code>TZ</code> environment variable.
883 While an administrator can "do everything in <abbr>UT</abbr>" to
884 get around the problem, doing so is inconvenient and precludes
885 handling daylight saving time shifts - as might be required to
886 limit phone calls to off-peak hours.)
676 In POSIX, there's no tamper-proof way for a process to learn the
677 system's best idea of local wall clock. (This is important for
678 applications that an administrator wants used only at certain
679 times &ndash;
680 without regard to whether the user has fiddled the TZ environment
681 variable. While an administrator can "do everything in UTC" to get
682 around the problem, doing so is inconvenient and precludes handling
683 daylight saving time shifts - as might be required to limit phone
684 calls to off-peak hours.)
887 </li>
888 <li>
685 </li>
686 <li>
889 POSIX provides no convenient and efficient way to determine
890 the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
891 timestamps, particularly for <code><abbr>tz</abbr></code> regions
892 that do not fit into the POSIX model.
687 POSIX provides no convenient and efficient way to determine the UT
688 offset and time zone abbreviation of arbitrary timestamps,
689 particularly for time zone settings that do not fit into the
690 POSIX model.
893 </li>
894 <li>
691 </li>
692 <li>
895 POSIX requires that systems ignore leap seconds.
693 POSIX requires that systems ignore leap seconds.
896 </li>
897 <li>
694 </li>
695 <li>
898 The <code><abbr>tz</abbr></code> code attempts to support all the
899 <code>time_t</code> implementations allowed by POSIX.
900 The <code>time_t</code> type represents a nonnegative count of seconds
901 since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds.
902 In practice, <code>time_t</code> is usually a signed 64- or 32-bit
903 integer; 32-bit signed <code>time_t</code> values stop working after
904 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these
905 days typically use a signed 64-bit integer.
906 Unsigned 32-bit integers are used on one or two platforms, and 36-bit
907 and 40-bit integers are also used occasionally.
908 Although earlier POSIX versions allowed <code>time_t</code> to be a
909 floating-point type, this was not supported by any practical systems,
910 and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both
911 require <code>time_t</code> to be an integer type.
696 The tz code attempts to support all the <code>time_t</code>
697 implementations allowed by POSIX. The <code>time_t</code>
698 type represents a nonnegative count of
699 seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds.
700 In practice, <code>time_t</code> is usually a signed 64- or
701 32-bit integer; 32-bit signed <code>time_t</code> values stop
702 working after 2038-01-19 03:14:07 UTC, so
703 new implementations these days typically use a signed 64-bit integer.
704 Unsigned 32-bit integers are used on one or two platforms,
705 and 36-bit and 40-bit integers are also used occasionally.
706 Although earlier POSIX versions allowed <code>time_t</code> to be a
707 floating-point type, this was not supported by any practical
708 systems, and POSIX.1-2013 and the tz code both
709 require <code>time_t</code>
710 to be an integer type.
912 </li>
913</ul>
711 </li>
712</ul>
914
915<h3 id="POSIX-extensions">Extensions to POSIX in the
916<code><abbr>tz</abbr></code> code</h3>
713<p>
714These are the extensions that have been made to the POSIX functions:
715</p>
917<ul>
918 <li>
919 <p>
716<ul>
717 <li>
718 <p>
920 The <code>TZ</code> environment variable is used in generating
921 the name of a binary file from which time-related information is read
922 (or is interpreted �� la POSIX); <code>TZ</code> is no longer
923 constrained to be a three-letter time zone
924 abbreviation followed by a number of hours and an optional three-letter
925 daylight time zone abbreviation.
926 The daylight saving time rules to be used for a
927 particular <code><abbr>tz</abbr></code> region are encoded in the
928 binary file; the format of the file
929 allows U.S., Australian, and other rules to be encoded, and
930 allows for situations where more than two time zone
931 abbreviations are used.
719 The TZ environment variable is used in generating the name of a file
720 from which time zone information is read (or is interpreted a la
721 POSIX); TZ is no longer constrained to be a three-letter time zone
722 name followed by a number of hours and an optional three-letter
723 daylight time zone name. The daylight saving time rules to be used
724 for a particular time zone are encoded in the time zone file;
725 the format of the file allows U.S., Australian, and other rules to be
726 encoded, and allows for situations where more than two time zone
727 abbreviations are used.
932 </p>
933 <p>
728 </p>
729 <p>
934 It was recognized that allowing the <code>TZ</code> environment
935 variable to take on values such as '<code>America/New_York</code>'
936 might cause "old" programs (that expect <code>TZ</code> to have a
937 certain form) to operate incorrectly; consideration was given to using
938 some other environment variable (for example, <code>TIMEZONE</code>)
939 to hold the string used to generate the binary file's name.
940 In the end, however, it was decided to continue using
941 <code>TZ</code>: it is widely used for time zone purposes;
942 separately maintaining both <code>TZ</code>
943 and <code>TIMEZONE</code> seemed a nuisance; and systems where
944 "new" forms of <code>TZ</code> might cause problems can simply
945 use <code>TZ</code> values such as "<code>EST5EDT</code>" which
946 can be used both by "new" programs (�� la POSIX) and "old"
947 programs (as zone names and offsets).
730 It was recognized that allowing the TZ environment variable to
731 take on values such as '<code>America/New_York</code>' might
732 cause "old" programs
733 (that expect TZ to have a certain form) to operate incorrectly;
734 consideration was given to using some other environment variable
735 (for example, TIMEZONE) to hold the string used to generate the
736 time zone information file name. In the end, however, it was decided
737 to continue using TZ: it is widely used for time zone purposes;
738 separately maintaining both TZ and TIMEZONE seemed a nuisance;
739 and systems where "new" forms of TZ might cause problems can simply
740 use TZ values such as "<code>EST5EDT</code>" which can be used both by
741 "new" programs (a la POSIX) and "old" programs (as zone names and
742 offsets).
948 </p>
743 </p>
949 </li>
950 <li>
951 The code supports platforms with a <abbr>UT</abbr> offset member
952 in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>.
953 </li>
954 <li>
955 The code supports platforms with a time zone abbreviation member in
956 <code>struct tm</code>, e.g., <code>tm_zone</code>.
957 </li>
958 <li>
959 Functions <code>tzalloc</code>, <code>tzfree</code>,
960 <code>localtime_rz</code>, and <code>mktime_z</code> for
961 more-efficient thread-safe applications that need to use multiple
962 time zone rulesets.
963 The <code>tzalloc</code> and <code>tzfree</code> functions
964 allocate and free objects of type <code>timezone_t</code>,
965 and <code>localtime_rz</code> and <code>mktime_z</code> are
966 like <code>localtime_r</code> and <code>mktime</code> with an
967 extra <code>timezone_t</code> argument.
968 The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>.
969 </li>
970 <li>
971 A function <code>tzsetwall</code> has been added to arrange for the
972 system's best approximation to local wall clock time to be delivered
973 by subsequent calls to <code>localtime</code>.
974 Source code for portable applications that "must" run on local wall
975 clock time should call <code>tzsetwall</code>;
976 if such code is moved to "old" systems that don't
977 provide <code>tzsetwall</code>, you won't be able to generate an
978 executable program.
979 (These functions also arrange for local wall clock time to
980 be used if <code>tzset</code> is called &ndash; directly or
981 indirectly &ndash; and there's no <code>TZ</code> environment
982 variable; portable applications should not, however, rely on this
983 behavior since it's not the way SVR2 systems behave.)
984 </li>
985 <li>
986 Negative <code>time_t</code> values are supported, on systems
987 where <code>time_t</code> is signed.
988 </li>
989 <li>
990 These functions can account for leap seconds, thanks to Bradley White.
991 </li>
744
745

  • 746 The code supports platforms with a UT offset member
    747 in <code>struct tm</code>,
    748 e.g., <code>tm_gmtoff</code>.
    749</li>
    750<li>
    751 The code supports platforms with a time zone abbreviation member in
    752 <code>struct tm</code>, e.g., <code>tm_zone</code>.
    753</li>
    754<li>
    755 Since the TZ environment variable can now be used to control time
    756 conversion, the <code>daylight</code>
    757 and <code>timezone</code> variables are no longer needed.
    758 (These variables are defined and set by <code>tzset</code>;
    759 however, their values will not be used
    760 by <code>localtime</code>.)
    761</li>
    762<li>
    763 Functions <code>tzalloc</code>, <code>tzfree</code>,
    764 <code>localtime_rz</code>, and <code>mktime_z</code> for
    765 more-efficient thread-safe applications that need to use
    766 multiple time zones. The <code>tzalloc</code>
    767 and <code>tzfree</code> functions allocate and free objects of
    768 type <code>timezone_t</code>, and <code>localtime_rz</code>
    769 and <code>mktime_z</code> are like <code>localtime_r</code>
    770 and <code>mktime</code> with an extra
    771 <code>timezone_t</code> argument. The functions were inspired
    772 by NetBSD.
    773</li>
    774<li>
    775 A function <code>tzsetwall</code> has been added to arrange
    776 for the system's
    777 best approximation to local wall clock time to be delivered by
    778 subsequent calls to <code>localtime</code>. Source code for portable
    779 applications that "must" run on local wall clock time should call
    780 <code>tzsetwall</code>; if such code is moved to "old" systems that don't
    781 provide tzsetwall, you won't be able to generate an executable program.
    782 (These time zone functions also arrange for local wall clock time to be
    783 used if tzset is called &ndash; directly or indirectly &ndash;
    784 and there's no TZ
    785 environment variable; portable applications should not, however, rely
    786 on this behavior since it's not the way SVR2 systems behave.)
    787</li>
    788<li>
    789 Negative <code>time_t</code> values are supported, on systems
    790 where <code>time_t</code> is signed.
    791</li>
    792<li>
    793 These functions can account for leap seconds, thanks to Bradley White.
    794</li>
    		•
    992</ul>
    		795</ul>
    993
    994<h3 id="vestigial">POSIX features no longer needed</h3>
    995<p>
    		796<p>
    996POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a>
    997define some <a href="https://en.wikipedia.org/wiki/API"><abbr
    998title="application programming interface">API</abbr>s</a> that are vestigial:
    999they are not needed, and are relics of a too-simple model that does
    1000not suffice to handle many real-world timestamps.
    1001Although the <code><abbr>tz</abbr></code> code supports these
    1002vestigial <abbr>API</abbr>s for backwards compatibility, they should
    1003be avoided in portable applications.
    1004The vestigial <abbr>API</abbr>s are:
    		797Points of interest to folks with other systems:
    1005</p>
    1006<ul>
    1007 <li>
    		798</p>
    799<ul>
    800 <li>
    1008 The POSIX <code>tzname</code> variable does not suffice and is no
    1009 longer needed.
    1010 To get a timestamp's time zone abbreviation, consult
    1011 the <code>tm_zone</code> member if available; otherwise,
    1012 use <code>strftime</code>'s <code>"%Z"</code> conversion
    1013 specification.
    		801 Code compatible with this package is already part of many platforms,
    802 including GNU/Linux, Android, the BSDs, Chromium OS, Cygwin, AIX, iOS,
    803 BlackBery 10, macOS, Microsoft Windows, OpenVMS, and Solaris.
    804 On such hosts, the primary use of this package
    805 is to update obsolete time zone rule tables.
    806 To do this, you may need to compile the time zone compiler
    807 '<code>zic</code>' supplied with this package instead of using
    808 the system '<code>zic</code>', since the format
    809 of <code>zic</code>'s input is occasionally extended, and a
    810 platform may still be shipping an older <code>zic</code>.
    1014 </li>
    1015 <li>
    		811 </li>
    812 <li>
    1016 The POSIX <code>daylight</code> and <code>timezone</code>
    1017 variables do not suffice and are no longer needed.
    1018 To get a timestamp's <abbr>UT</abbr> offset, consult
    1019 the <code>tm_gmtoff</code> member if available; otherwise,
    1020 subtract values returned by <code>localtime</code>
    1021 and <code>gmtime</code> using the rules of the Gregorian calendar,
    1022 or use <code>strftime</code>'s <code>"%z"</code> conversion
    1023 specification if a string like <code>"+0900"</code> suffices.
    		813 The UNIX Version 7 <code>timezone</code> function is not
    814 present in this package;
    815 it's impossible to reliably map timezone's arguments (a "minutes west
    816 of GMT" value and a "daylight saving time in effect" flag) to a
    817 time zone abbreviation, and we refuse to guess.
    818 Programs that in the past used the timezone function may now examine
    819 <code>localtime(&amp;clock)-&gt;tm_zone</code>
    820 (if <code>TM_ZONE</code> is defined) or
    821 <code>tzname[localtime(&amp;clock)-&gt;tm_isdst]</code>
    822 (if <code>HAVE_TZNAME</code> is defined)
    823 to learn the correct time zone abbreviation to use.
    1024 </li>
    1025 <li>
    		824 </li>
    825 <li>
    1026 The <code>tm_isdst</code> member is almost never needed and most of
    1027 its uses should be discouraged in favor of the abovementioned
    1028 <abbr>API</abbr>s.
    1029 Although it can still be used in arguments to
    1030 <code>mktime</code> to disambiguate timestamps near
    1031 a <abbr>DST</abbr> transition when the clock jumps back, this
    1032 disambiguation does not work when standard time itself jumps back,
    1033 which can occur when a location changes to a time zone with a
    1034 lesser <abbr>UT</abbr> offset.
    		826 The 4.2BSD <code>gettimeofday</code> function is not used in
    827 this package.
    828 This formerly let users obtain the current UTC offset and DST flag,
    829 but this functionality was removed in later versions of BSD.
    1035 </li>
    		830 </li>
    1036</ul>
    1037
    1038<h3 id="other-portability">Other portability notes</h3>
    1039<ul>
    1040 <li>
    		831 <li>
    1041 The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition
    1042 UNIX</a> <code>timezone</code> function is not present in this
    1043 package; it's impossible to reliably map <code>timezone</code>'s
    1044 arguments (a "minutes west of <abbr>GMT</abbr>" value and a
    1045 "daylight saving time in effect" flag) to a time zone
    1046 abbreviation, and we refuse to guess.
    1047 Programs that in the past used the <code>timezone</code> function
    1048 may now examine <code>localtime(&amp;clock)-&gt;tm_zone</code>
    1049 (if <code>TM_ZONE</code> is defined) or
    1050 <code>tzname[localtime(&amp;clock)-&gt;tm_isdst]</code>
    1051 (if <code>HAVE_TZNAME</code> is defined) to learn the correct time
    1052 zone abbreviation to use.
    		832 In SVR2, time conversion fails for near-minimum or near-maximum
    833 <code>time_t</code> values when doing conversions for places
    834 that don't use UT.
    835 This package takes care to do these conversions correctly.
    836 A comment in the source code tells how to get compatibly wrong
    837 results.
    1053 </li>
    		838 </li>
    1054 <li>
    1055 The <abbr>4.2BSD</abbr> <code>gettimeofday</code> function is not
    1056 used in this package.
    1057 This formerly let users obtain the current <abbr>UTC</abbr> offset
    1058 and <abbr>DST</abbr> flag, but this functionality was removed in
    1059 later versions of <abbr>BSD</abbr>.
    1060 </li>
    1061 <li>
    1062 In <abbr>SVR2</abbr>, time conversion fails for near-minimum or
    1063 near-maximum <code>time_t</code> values when doing conversions
    1064 for places that don't use <abbr>UT</abbr>.
    1065 This package takes care to do these conversions correctly.
    1066 A comment in the source code tells how to get compatibly wrong
    1067 results.
    1068 </li>
    1069 <li>
    1070 The functions that are conditionally compiled
    1071 if <code>STD_INSPIRED</code> is defined should, at this point, be
    1072 looked on primarily as food for thought.
    1073 They are not in any sense "standard compatible" &ndash; some are
    1074 not, in fact, specified in <em>any</em> standard.
    1075 They do, however, represent responses of various authors to
    1076 standardization proposals.
    1077 </li>
    1078 <li>
    1079 Other time conversion proposals, in particular the one developed
    1080 by folks at Hewlett Packard, offer a wider selection of functions
    1081 that provide capabilities beyond those provided here.
    1082 The absence of such functions from this package is not meant to
    1083 discourage the development, standardization, or use of such
    1084 functions.
    1085 Rather, their absence reflects the decision to make this package
    1086 contain valid extensions to POSIX, to ensure its broad
    1087 acceptability.
    1088 If more powerful time conversion functions can be standardized, so
    1089 much the better.
    1090 </li>
    1091</ul>
    		839</ul>
    1092</section>
    		840<p>
    841The functions that are conditionally compiled
    842if <code>STD_INSPIRED</code> is defined
    843should, at this point, be looked on primarily as food for thought. They are
    844not in any sense "standard compatible" &ndash; some are not, in fact,
    845specified in <em>any</em> standard. They do, however, represent responses of
    846various authors to
    847standardization proposals.
    848</p>
    1093
    		849
    1094<section>
    1095 <h2 id="stability">Interface stability</h2>
    1096<p>
    		850<p>
    1097The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
    		851Other time conversion proposals, in particular the one developed by folks at
    852Hewlett Packard, offer a wider selection of functions that provide capabilities
    853beyond those provided here. The absence of such functions from this package
    854is not meant to discourage the development, standardization, or use of such
    855functions. Rather, their absence reflects the decision to make this package
    856contain valid extensions to POSIX, to ensure its broad acceptability. If
    857more powerful time conversion functions can be standardized, so much the
    858better.
    1098</p>
    		859</p>
    860 </section>
    1099
    		861
    862
    863 <section>
    864 <h2 id="stability">Interface stability</h2>
    865<p>
    866The tz code and data supply the following interfaces:
    867</p>
    1100<ul>
    1101 <li>
    		868<ul>
    869 <li>
    1102 A set of <code><abbr>tz</abbr></code> region names as per
    1103 "<a href="#naming">Names of time zone rulesets</a>" above.
    		870 A set of zone names as per "<a href="#naming">Names of time zone
    871 rules</a>" above.
    1104 </li>
    1105 <li>
    		872 </li>
    873 <li>
    1106 Library functions described in "<a href="#functions">Time and date
    1107 functions</a>" above.
    		874 Library functions described in "Time and date
    875     functions" above.
    1108 </li>
    1109 <li>
    		876 </li>
    877 <li>
    1110 The programs <code>tzselect</code>, <code>zdump</code>,
    1111 and <code>zic</code>, documented in their man pages.
    		878 The programs tzselect, zdump,
    879 and zic, documented in their man pages.
    1112 </li>
    1113 <li>
    		880 </li>
    881 <li>
    1114 The format of <code>zic</code> input files, documented in
    1115 the <code>zic</code> man page.
    		882 The format of zic input files, documented in
    883 the zic man page.
    1116 </li>
    1117 <li>
    		884 </li>
    885 <li>
    1118 The format of <code>zic</code> output files, documented in
    1119 the <code>tzfile</code> man page.
    		886 The format of zic output files, documented in
    887 the tzfile man page.
    1120 </li>
    1121 <li>
    		888 </li>
    889 <li>
    1122 The format of zone table files, documented in <code>zone1970.tab</code>.
    		890 The format of zone table files, documented in zone1970.tab.
    1123 </li>
    1124 <li>
    		891 </li>
    892 <li>
    1125 The format of the country code file, documented in <code>iso3166.tab</code>.
    		893 The format of the country code file, documented in iso3166.tab.
    1126 </li>
    1127 <li>
    		894 </li>
    895 <li>
    1128 The version number of the code and data, as the first line of
    1129 the text file '<code>version</code>' in each release.
    		896 The version number of the code and data, as the first line of
    897 the text file 'version' in each release.
    1130 </li>
    1131</ul>
    		898 </li>
    899</ul>
    1132
    1133<p>
    1134Interface changes in a release attempt to preserve compatibility with
    		900<p>
    901Interface changes in a release attempt to preserve compatibility with
    1135recent releases.
    1136For example, <code><abbr>tz</abbr></code> data files typically do not
    1137rely on recently-added <code>zic</code> features, so that users can
    1138run older <code>zic</code> versions to process newer data files.
    1139<a href="tz-link.html#download">Downloading
    1140the <code><abbr>tz</abbr></code> database</a> describes how releases
    1141are tagged and distributed.
    		902recent releases. For example, tz data files typically do not rely on
    903recently-added <code>zic</code> features, so that users can run
    904older <code>zic</code> versions to process newer data
    905files. <a href="tz-link.htm">Sources for time zone and daylight
    906saving time data</a> describes how
    907releases are tagged and distributed.
    1142</p>
    1143
    1144<p>
    		908</p>
    909
    910<p>
    1145Interfaces not listed above are less stable.
    1146For example, users should not rely on particular <abbr>UT</abbr>
    1147offsets or abbreviations for timestamps, as data entries are often
    1148based on guesswork and these guesses may be corrected or improved.
    		911Interfaces not listed above are less stable. For example, users
    912should not rely on particular UT offsets or abbreviations for
    913timestamps, as data entries are often based on guesswork and these
    914guesses may be corrected or improved.
    1149</p>
    		915</p>
    1150
    		916 </section>
    1151
    		917
    1152<section>
    1153 <h2 id="calendar">Calendrical issues</h2>
    		918
    919 <section>
    920 <h2 id="calendar">Calendrical issues</h2>
    1154<p>
    1155Calendrical issues are a bit out of scope for a time zone database,
    1156but they indicate the sort of problems that we would run into if we
    		921<p>
    922Calendrical issues are a bit out of scope for a time zone database,
    923but they indicate the sort of problems that we would run into if we
    1157extended the time zone database further into the past.
    1158An excellent resource in this area is Nachum Dershowitz and Edward M.
    1159Reingold, <cite><a
    1160href="https://www.cs.tau.ac.il/~nachum/calendar-book/third-edition/">Calendrical
    		924extended the time zone database further into the past. An excellent
    925resource in this area is Nachum Dershowitz and Edward M. Reingold,
    926<cite><a href="https://www.cs.tau.ac.il/~nachum/calendar-book/third-edition/">Calendrical
    1161Calculations: Third Edition</a></cite>, Cambridge University Press (2008).
    		927Calculations: Third Edition</a></cite>, Cambridge University Press (2008).
    1162Other information and sources are given in the file '<code>calendars</code>'
    1163in the <code><abbr>tz</abbr></code> distribution.
    1164They sometimes disagree.
    		928Other information and sources are given in the file '<samp>calendars</samp>'
    929in the tz distribution. They sometimes disagree.
    1165</p>
    		930</p>
    1166
    		931 </section>
    1167
    		932
    1168<section>
    1169 <h2 id="planets">Time and time zones on other planets</h2>
    		933
    934 <section>
    935 <h2 id="planets">Time and time zones on other planets</h2>
    1170<p>
    		936<p>
    1171Some people's work schedules
    1172use <a href="https://en.wikipedia.org/wiki/Timekeeping on Mars">Mars time</a>.
    1173Jet Propulsion Laboratory (JPL) coordinators have kept Mars time on
    1174and off at least since 1997 for the
    1175<a href="https://en.wikipedia.org/wiki/Mars_Pathfinder#End_of_mission">Mars
    1176Pathfinder</a> mission.
    1177Some of their family members have also adapted to Mars time.
    1178Dozens of special Mars watches were built for JPL workers who kept
    1179Mars time during the Mars Exploration Rovers mission (2004).
    1180These timepieces look like normal Seikos and Citizens but use Mars
    1181seconds rather than terrestrial seconds.
    		937Some people's work schedules use Mars time. Jet Propulsion Laboratory
    938(JPL) coordinators have kept Mars time on and off at least since 1997
    939for the Mars Pathfinder mission. Some of their family members have
    940also adapted to Mars time. Dozens of special Mars watches were built
    941for JPL workers who kept Mars time during the Mars Exploration
    942Rovers mission (2004). These timepieces look like normal Seikos and
    943Citizens but use Mars seconds rather than terrestrial seconds.
    1182</p>
    1183
    1184<p>
    1185A Mars solar day is called a "sol" and has a mean period equal to
    		944</p>
    945
    946<p>
    947A Mars solar day is called a "sol" and has a mean period equal to
    1186about 24 hours 39 minutes 35.244 seconds in terrestrial time.
    1187It is divided into a conventional 24-hour clock, so each Mars second
    1188equals about 1.02749125 terrestrial seconds.
    		948about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is
    949divided into a conventional 24-hour clock, so each Mars second equals
    950about 1.02749125 terrestrial seconds.
    1189</p>
    1190
    1191<p>
    		951</p>
    952
    953<p>
    1192The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime
    1193meridian</a> of Mars goes through the center of the crater
    1194<a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in
    1195honor of the British astronomer who built the Greenwich telescope that
    1196defines Earth's prime meridian.
    1197Mean solar time on the Mars prime meridian is
    1198called <a href="https://en.wikipedia.org/wiki/Mars_Coordinated_Time">Mars
    1199Coordinated Time (<abbr>MTC</abbr>)</a>.
    		954The prime meridian of Mars goes through the center of the crater
    955Airy-0, named in honor of the British astronomer who built the
    956Greenwich telescope that defines Earth's prime meridian. Mean solar
    957time on the Mars prime meridian is called Mars Coordinated Time (MTC).
    1200</p>
    1201
    1202<p>
    1203Each landed mission on Mars has adopted a different reference for
    1204solar time keeping, so there is no real standard for Mars time zones.
    		958</p>
    959
    960<p>
    961Each landed mission on Mars has adopted a different reference for
    962solar time keeping, so there is no real standard for Mars time zones.
    1205For example, the
    1206<a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars
    1207Exploration Rover</a> project (2004) defined two time zones "Local
    1208Solar Time A" and "Local Solar Time B" for its two missions, each zone
    1209designed so that its time equals local true solar time at
    1210approximately the middle of the nominal mission.
    1211Such a "time zone" is not particularly suited for any application
    1212other than the mission itself.
    		963For example, the Mars Exploration Rover project (2004) defined two
    964time zones "Local Solar Time A" and "Local Solar Time B" for its two
    965missions, each zone designed so that its time equals local true solar
    966time at approximately the middle of the nominal mission. Such a "time
    967zone" is not particularly suited for any application other than the
    968mission itself.
    1213</p>
    1214
    1215<p>
    1216Many calendars have been proposed for Mars, but none have achieved
    		969</p>
    970
    971<p>
    972Many calendars have been proposed for Mars, but none have achieved
    1217wide acceptance.
    1218Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
    		973wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a
    1219sequential count of Mars solar days elapsed since about 1873-12-29
    		974sequential count of Mars solar days elapsed since about 1873-12-29
    122012:00 <abbr>GMT</abbr>.
    		97512:00 GMT.
    1221</p>
    1222
    1223<p>
    1224In our solar system, Mars is the planet with time and calendar most
    		976</p>
    977
    978<p>
    979In our solar system, Mars is the planet with time and calendar most
    1225like Earth's.
    1226On other planets, Sun-based time and calendars would work quite
    1227differently.
    1228For example, although Mercury's
    1229<a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal
    1230rotation period</a> is 58.646 Earth days, Mercury revolves around the
    1231Sun so rapidly that an observer on Mercury's equator would see a
    1232sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a
    1233Mercury day.
    1234Venus is more complicated, partly because its rotation is slightly
    1235<a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>:
    1236its year is 1.92 of its days.
    1237Gas giants like Jupiter are trickier still, as their polar and
    1238equatorial regions rotate at different rates, so that the length of a
    1239day depends on latitude.
    1240This effect is most pronounced on Neptune, where the day is about 12
    1241hours at the poles and 18 hours at the equator.
    		980like Earth's. On other planets, Sun-based time and calendars would
    981work quite differently. For example, although Mercury's sidereal
    982rotation period is 58.646 Earth days, Mercury revolves around the Sun
    983so rapidly that an observer on Mercury's equator would see a sunrise
    984only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a Mercury
    985day. Venus is more complicated, partly because its rotation is
    986slightly retrograde: its year is 1.92 of its days. Gas giants like
    987Jupiter are trickier still, as their polar and equatorial regions
    988rotate at different rates, so that the length of a day depends on
    989latitude. This effect is most pronounced on Neptune, where the day is
    990about 12 hours at the poles and 18 hours at the equator.
    1242</p>
    1243
    1244<p>
    		991</p>
    992
    993<p>
    1245Although the <code><abbr>tz</abbr></code> database does not support
    1246time on other planets, it is documented here in the hopes that support
    1247will be added eventually.
    		994Although the tz database does not support time on other planets, it is
    995documented here in the hopes that support will be added eventually.
    1248</p>
    1249
    1250<p>
    		996</p>
    997
    998<p>
    1251Sources for time on other planets:
    		999Sources:
    1252</p>
    		1000</p>
    1253
    1254<ul>
    1255 <li>
    		1001<ul>
    1002 <li>
    1256 Michael Allison and Robert Schmunk,
    1257 "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
    1258 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>"
    1259 (2015-06-30).
    		1003Michael Allison and Robert Schmunk,
    1004"Technical
    1005    Notes on Mars Solar Time as Adopted by the Mars24 Sunclock"
    1006(2012-08-08).
    1260 </li>
    1261 <li>
    		1007 </li>
    1008 <li>
    1262 Jia-Rui Chong,
    1263 "<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays
    1264 Fit for a Martian</a>", <cite>Los Angeles Times</cite>
    1265 (2004-01-14), pp A1, A20-A21.
    		1009Jia-Rui Chong,
    1010"Workdays
    1011    Fit for a Martian</a>", Los Angeles Times
    1012(2004-01-14), pp A1, A20-A21.
    1266 </li>
    1267 <li>
    		1013 </li>
    1014 <li>
    1268 Tom Chmielewski,
    1269 "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet
    1270 Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26)
    		1015Tom Chmielewski,
    1016"Jet
    1017    Lag Is Worse on Mars</a>", The Atlantic (2015-02-26)
    1271 </li>
    1272 <li>
    		1018 </li>
    1019 <li>
    1273 Matt Williams,
    1274 "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How
    1275 long is a day on the other planets of the solar system?</a>"
    1276 (2017-04-27).
    		1020Matt Williams,
    1021"How
    1022    long is a day on the other planets of the solar system?"
    1023(2017-04-27).
    1277 </li>
    1278</ul>
    		1024 </li>
    1025</ul>
    1279
    		1026 </section>
    1280
    		1027
    1281

    1282

    1283 This file is in the public domain, so clarified as of 2009-05-17 by
    1284 Arthur David Olson.
    1285

    		1028 <footer>
    1029 <hr>
    1030This file is in the public domain, so clarified as of 2009-05-17 by
    1031Arthur David Olson.
    1032 </footer>
    1286</body>
    1287</html>
    		1033</body>
    1034</html>