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16 <h1>Theory and pragmatics of the tz code and data</h1>
8<h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1>
17 <h3>Outline</h3>
18 <nav>
19 <ul>
9 <h3>Outline</h3>
10 <nav>
11 <ul>
20 <li><a href="#scope">Scope of the tz database</a></li>
21 <li><a href="#naming">Names of time zone rules</a></li>
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>
22 <li><a href="#abbreviations">Time zone abbreviations</a></li>
15 <li><a href="#abbreviations">Time zone abbreviations</a></li>
23 <li><a href="#accuracy">Accuracy of the tz database</a></li>
16 <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code>
17 database</a></li>
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
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
31
32 <section>
33 <h2 id="scope">Scope of the tz database</h2>
25<section>
26 <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2>
34<p>
27<p>
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.
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.
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.
55Athough some information outside the scope of the database is
56collected in a file <code>backzone</code> that is distributed along
57with the database proper, this file is less reliable and does not
58necessarily follow database guidelines.
59</p>
60
61<p>
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.
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>
62As described below, reference source code for using the tz database is
63also available. The tz code is upwards compatible with POSIX, an
64international standard for UNIX-like systems. As of this writing, the
65current edition of POSIX is:
66 <a href="http://pubs.opengroup.org/onlinepubs/9699919799/">
67 The Open Group Base Specifications Issue 7</a>,
68 IEEE Std 1003.1-2008, 2016 Edition.
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.
69</p>
84</p>
70 </section>
71
85
86</section>
72
87
73
74 <section>
75 <h2 id="naming">Names of time zone rules</h2>
88<section>
89 <h2 id="naming">Names of time zone rulesets</h2>
76<p>
90<p>
77Each of the database's time zone rules has a unique name.
91Each <code><abbr>tz</abbr></code> region has a unique name that
92corresponds to a set of time zone rules.
78Inexperienced users are not expected to select these names unaided.
79Distributors should provide documentation and/or a simple selection
80interface that explains the names; for one example, see the 'tzselect'
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'
81program in the tz code. The
82Unicode Common Locale Data
83Repository contains data that may be useful for other
84selection interfaces.
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.
85</p>
86
87<p>
100</p>
101
102<p>
88The time zone rule naming conventions attempt to strike a balance
103The naming conventions attempt to strike a balance
89among the following goals:
90</p>
104among the following goals:
105</p>
106
91<ul>
92 <li>
107<ul>
108 <li>
93 Uniquely identify every region where clocks have agreed since 1970.
94 This is essential for the intended use: static clocks keeping local
95 civil time.
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.
96 </li>
97 <li>
112 </li>
113 <li>
98 Indicate to experts where that region is.
114 Indicate to experts where that region is.
99 </li>
100 <li>
115 </li>
116 <li>
101 Be robust in the presence of political changes. For example, names
102 of countries are ordinarily not used, to avoid incompatibilities
103 when countries change their name (e.g. Zaire&rarr;Congo) or when
104 locations change countries (e.g. Hong Kong from UK colony to
105 China).
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).
106 </li>
107 <li>
122 </li>
123 <li>
108 Be portable to a wide variety of implementations.
124 Be portable to a wide variety of implementations.
109 </li>
110 <li>
125 </li>
126 <li>
111 Use a consistent naming conventions over the entire world.
127 Use a consistent naming conventions over the entire world.
112 </li>
113</ul>
128 </li>
129</ul>
130
114<p>
131<p>
115Names normally have the
116form <var>AREA</var><code>/</code><var>LOCATION</var>,
117where <var>AREA</var> is the name of a continent or ocean,
118and <var>LOCATION</var> is the name of a specific
119location within that region. North and South America share the same
120area, '<code>America</code>'. Typical names are
121'<code>Africa/Cairo</code>', '<code>America/New_York</code>', and
122'Pacific/Honolulu'.
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>'.
123</p>
124
125<p>
140</p>
141
142<p>
126Here are the general rules used for choosing location names,
143Here are the general guidelines used for
144choosing <code><abbr>tz</abbr></code> region names,
127in decreasing order of importance:
128</p>
145in decreasing order of importance:
146</p>
147
129<ul>
130 <li>
148<ul>
149 <li>
131 Use only valid POSIX file name components (i.e., the parts of
132 names other than '<code>/</code>'). Do not use the file name
133 components '<code>.</code>' and '<code>..</code>'.
134 Within a file name component,
135 use only ASCII letters, '<code>.</code>',
136 '<code>-</code>' and '<code>_</code>'. Do not use
137 digits, as that might create an ambiguity with POSIX
138 TZ strings. A file name component must not exceed 14
139 characters or start with '<code>-</code>'. E.g.,
140 prefer '<code>Brunei</code>' to
141 '<code>Bandar_Seri_Begawan</code>'. Exceptions: see
142 the discussion
143 of legacy names below.
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.
144 </li>
145 <li>
164 </li>
165 <li>
146 A name must not be empty, or contain '<code>//</code>', or
147 start or end with '<code>/</code>'.
166 A name must not be empty, or contain '<code>//</code>', or
167 start or end with '<code>/</code>'.
148 </li>
149 <li>
168 </li>
169 <li>
150 Do not use names that differ only in case. Although the reference
151 implementation is case-sensitive, some other implementations
152 are not, and they would mishandle names differing only in case.
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.
153 </li>
154 <li>
174 </li>
175 <li>
155 If one name <var>A</var> is an initial prefix of another
156 name <var>AB</var> (ignoring case), then <var>B</var>
157 must not start with '<code>/</code>', as a
158 regular file cannot have
159 the same name as a directory in POSIX. For example,
160 '<code>America/New_York</code>' precludes
161 '<code>America/New_York/Bronx</code>'.
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>'.
162 </li>
163 <li>
182 </li>
183 <li>
164 Uninhabited regions like the North Pole and Bouvet Island
165 do not need locations, since local time is not defined there.
184 Uninhabited regions like the North Pole and Bouvet Island
185 do not need locations, since local time is not defined there.
166 </li>
167 <li>
186 </li>
187 <li>
168 There should typically be at least one name for each ISO 3166-1
169 officially assigned two-letter code for an inhabited country
170 or territory.
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.
171 </li>
172 <li>
193 </li>
194 <li>
173 If all the clocks in a region have agreed since 1970,
174 don't bother to include more than one location
175 even if subregions' clocks disagreed before 1970.
176 Otherwise these tables would become annoyingly large.
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.
177 </li>
178 <li>
199 </li>
200 <li>
179 If a name is ambiguous, use a less ambiguous alternative;
180 e.g. many cities are named San Jos�� and Georgetown, so
181 prefer '<code>Costa_Rica</code>' to '<code>San_Jose</code>' and '<code>Guyana</code>' to '<code>Georgetown</code>'.
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>'.
182 </li>
183 <li>
205 </li>
206 <li>
184 Keep locations compact. Use cities or small islands, not countries
185 or regions, so that any future time zone changes do not split
186 locations into different time zones. E.g. prefer
187 '<code>Paris</code>' to '<code>France</code>', since
188 France has had multiple time zones.
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>.
189 </li>
190 <li>
214 </li>
215 <li>
191 Use mainstream English spelling, e.g. prefer
192 '<code>Rome</code>' to '<code>Roma</code>', and prefer
193 '<code>Athens</code>' to the Greek
194 '<code>����������</code>' or the Romanized
195 '<code>Ath��na</code>'.
196 The POSIX file name restrictions encourage this rule.
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.
197 </li>
198 <li>
220 </li>
221 <li>
199 Use the most populous among locations in a zone,
200 e.g. prefer '<code>Shanghai</code>' to
201 '<code>Beijing</code>'. Among locations with
202 similar populations, pick the best-known location,
203 e.g. prefer '<code>Rome</code>' to '<code>Milan</code>'.
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>'.
204 </li>
205 <li>
228 </li>
229 <li>
206 Use the singular form, e.g. prefer '<code>Canary</code>' to '<code>Canaries</code>'.
230 Use the singular form, e.g., prefer '<code>Canary</code>' to
231 '<code>Canaries</code>'.
207 </li>
208 <li>
232 </li>
233 <li>
209 Omit common suffixes like '<code>_Islands</code>' and
210 '<code>_City</code>', unless that would lead to
211 ambiguity. E.g. prefer '<code>Cayman</code>' to
212 '<code>Cayman_Islands</code>' and
213 '<code>Guatemala</code>' to
214 '<code>Guatemala_City</code>', but prefer
215 '<code>Mexico_City</code>' to '<code>Mexico</code>'
216 because the country
217 of Mexico has several time zones.
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>.
218 </li>
219 <li>
242 </li>
243 <li>
220 Use '<code>_</code>' to represent a space.
244 Use '<code>_</code>' to represent a space.
221 </li>
222 <li>
245 </li>
246 <li>
223 Omit '<code>.</code>' from abbreviations in names, e.g. prefer
224 '<code>St_Helena</code>' to '<code>St._Helena</code>'.
247 Omit '<code>.</code>' from abbreviations in names.
248 E.g., prefer '<code>St_Helena</code>' to '<code>St._Helena</code>'.
225 </li>
226 <li>
249 </li>
250 <li>
227 Do not change established names if they only marginally
228 violate the above rules. For example, don't change
229 the existing name '<code>Rome</code>' to
230 '<code>Milan</code>' merely because
231 Milan's population has grown to be somewhat greater
232 than Rome's.
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.
233 </li>
234 <li>
256 </li>
257 <li>
235 If a name is changed, put its old spelling in the
236 '<code>backward</code>' file.
237 This means old spellings will continue to work.
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.
238 </li>
239</ul>
240
241<p>
242The file '<code>zone1970.tab</code>' lists geographical locations used
261 </li>
262</ul>
263
264<p>
265The file '<code>zone1970.tab</code>' lists geographical locations used
243to name time
244zone rules. It is intended to be an exhaustive list of names for
245geographic regions as described above; this is a subset of the names
246in the data. Although a '<code>zone1970.tab</code>' location's longitude
247corresponds to its LMT offset with one hour for every 15&deg; east
248longitude, this relationship is not exact.
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.
249</p>
250
251<p>
252Older versions of this package used a different naming scheme,
253and these older names are still supported.
254See the file '<code>backward</code>' for most of these older names
255(e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
256The other old-fashioned names still supported are
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
257'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and '<code>EET</code>' (see the file '<code>europe</code>').
283'WET', 'CET', 'MET', and
284'<code>EET</code>' (see the file '<code>europe</code>').
258</p>
259
260<p>
261Older versions of this package defined legacy names that are
285</p>
286
287<p>
288Older versions of this package defined legacy names that are
262incompatible with the first rule of location names, but which are
263still supported. These legacy names are mostly defined in the file
264'<code>etcetera</code>'. Also, the file '<code>backward</code>' defines the legacy names
265'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', and the file '<code>northamerica</code>' defines the
266legacy names '<code>EST5EDT</code>', '<code>CST6CDT</code>', '<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
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>'.
267</p>
268
269<p>
298</p>
299
300<p>
270Excluding '<code>backward</code>' should not affect the other data. If
271'<code>backward</code>' is excluded, excluding '<code>etcetera</code>' should not affect the
272remaining data.
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.
273</p>
304</p>
305</section>
274
306
275
276 </section>
277 <section>
278 <h2 id="abbreviations">Time zone abbreviations</h2>
307<section>
308 <h2 id="abbreviations">Time zone abbreviations</h2>
279<p>
280When this package is installed, it generates time zone abbreviations
281like '<code>EST</code>' to be compatible with human tradition and POSIX.
309<p>
310When this package is installed, it generates time zone abbreviations
311like '<code>EST</code>' to be compatible with human tradition and POSIX.
282Here are the general rules used for choosing time zone abbreviations,
312Here are the general guidelines used for choosing time zone abbreviations,
283in decreasing order of importance:
313in decreasing order of importance:
314</p>
315
284<ul>
285 <li>
316<ul>
317 <li>
286 Use three to six characters that are ASCII alphanumerics or
287 '<code>+</code>' or '<code>-</code>'.
288 Previous editions of this database also used characters like
289 '<code> </code>' and '<code>?</code>', but these
290 characters have a special meaning to
291 the shell and cause commands like
292 '<code>set `date`</code>'
293 to have unexpected effects.
294 Previous editions of this rule required upper-case letters,
295 but the Congressman who introduced Chamorro Standard Time
296 preferred "ChST", so lower-case letters are now allowed.
297 Also, POSIX from 2001 on relaxed the rule to allow
298 '<code>-</code>', '<code>+</code>',
299 and alphanumeric characters from the portable character set
300 in the current locale. In practice ASCII alphanumerics and
301 '<code>+</code>' and '<code>-</code>' are safe in all locales.
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.
302
336
303 In other words, in the C locale the POSIX extended regular
304 expression <code>[-+[:alnum:]]{3,6}</code> should match
305 the abbreviation.
306 This guarantees that all abbreviations could have been
307 specified by a POSIX TZ string.
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>
308 </li>
309 <li>
344 </li>
345 <li>
310 Use abbreviations that are in common use among English-speakers,
311 e.g. 'EST' for Eastern Standard Time in North America.
312 We assume that applications translate them to other languages
313 as part of the normal localization process; for example,
314 a French application might translate 'EST' to 'HNE'.
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'.
315
351
316<p><small>These abbreviations (for standard/daylight/etc. time) are:
317ACST/ACDT Australian Central,
318AST/ADT/APT/AWT/ADDT Atlantic,
319AEST/AEDT Australian Eastern,
320AHST/AHDT Alaska-Hawaii,
321AKST/AKDT Alaska,
322AWST/AWDT Australian Western,
323BST/BDT Bering,
324CAT/CAST Central Africa,
325CET/CEST/CEMT Central European,
326ChST Chamorro,
327CST/CDT/CWT/CPT/CDDT Central [North America],
328CST/CDT China,
329GMT/BST/IST/BDST Greenwich,
330EAT East Africa,
331EST/EDT/EWT/EPT/EDDT Eastern [North America],
332EET/EEST Eastern European,
333GST Guam,
334HST/HDT Hawaii,
335HKT/HKST Hong Kong,
336IST India,
337IST/GMT Irish,
338IST/IDT/IDDT Israel,
339JST/JDT Japan,
340KST/KDT Korea,
341MET/MEST Middle European (a backward-compatibility alias for Central European),
342MSK/MSD Moscow,
343MST/MDT/MWT/MPT/MDDT Mountain,
344NST/NDT/NWT/NPT/NDDT Newfoundland,
345NST/NDT/NWT/NPT Nome,
346NZMT/NZST New Zealand through 1945,
347NZST/NZDT New Zealand 1946&ndash;present,
348PKT/PKST Pakistan,
349PST/PDT/PWT/PPT/PDDT Pacific,
350SAST South Africa,
351SST Samoa,
352WAT/WAST West Africa,
353WET/WEST/WEMT Western European,
354WIB Waktu Indonesia Barat,
355WIT Waktu Indonesia Timur,
356WITA Waktu Indonesia Tengah,
357YST/YDT/YWT/YPT/YDDT Yukon</small>.</p>
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>
358 </li>
359 <li>
397 </li>
398 <li>
360 For zones whose times are taken from a city's longitude, use the
361traditional <var>x</var>MT notation. The only abbreviation like this
362in current use is 'GMT'. The others are for timestamps before 1960,
363except that Monrovia Mean Time persisted until 1972. Typically,
364numeric abbreviations (e.g., '<code>-</code>004430' for MMT) would
365cause trouble here, as the numeric strings would exceed the POSIX length limit.
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>
366
409
367<p><small>These abbreviations are:
368AMT Amsterdam, Asunci��n, Athens;
369BMT Baghdad, Bangkok, Batavia, Bern, Bogot��, Bridgetown, Brussels, Bucharest;
370CMT Calamarca, Caracas, Chisinau, Col��n, Copenhagen, C��rdoba;
371DMT Dublin/Dunsink;
372EMT Easter;
373FFMT Fort-de-France;
374FMT Funchal;
375GMT Greenwich;
376HMT Havana, Helsinki, Horta, Howrah;
377IMT Irkutsk, Istanbul;
378JMT Jerusalem;
379KMT Kaunas, Kiev, Kingston;
380LMT Lima, Lisbon, local, Luanda;
381MMT Macassar, Madras, Mal��, Managua, Minsk, Monrovia, Montevideo, Moratuwa,
382 Moscow;
383PLMT Ph�� Li���n;
384PMT Paramaribo, Paris, Perm, Pontianak, Prague;
385PMMT Port Moresby;
386QMT Quito;
387RMT Rangoon, Riga, Rome;
388SDMT Santo Domingo;
389SJMT San Jos��;
390SMT Santiago, Simferopol, Singapore, Stanley;
391TBMT Tbilisi;
392TMT Tallinn, Tehran;
393WMT Warsaw</small>.</p>
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>
394
440
395<p><small>A few abbreviations also follow the pattern that
396GMT/BST established for time in the UK. They are:
397
398CMT/BST for Calamarca Mean Time and Bolivian Summer Time
3991890&ndash;1932, DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
4001880&ndash;1916, MMT/MST/MDST for Moscow 1880&ndash;1919, and RMT/LST
401for Riga Mean Time and Latvian Summer time 1880&ndash;1926.
402An extra-special case is SET for Swedish Time (<em>svensk
403normaltid</em>) 1879&ndash;1899, 3&deg; west of the Stockholm
404Observatory.</small></p>
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>
405 </li>
406 <li>
455 </li>
456 <li>
407 Use 'LMT' for local mean time of locations before the introduction
408 of standard time; see "<a href="#scope">Scope of the
409 tz database</a>".
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>".
410 </li>
411 <li>
460 </li>
461 <li>
412 If there is no common English abbreviation, use numeric offsets like
413 <code>-</code>05 and <code>+</code>0830 that are
414 generated by zic's <code>%z</code> notation.
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.
415 </li>
416 <li>
465 </li>
466 <li>
417 Use current abbreviations for older timestamps to avoid confusion.
418 For example, in 1910 a common English abbreviation for UT +01
419 in central Europe was 'MEZ' (short for both "Middle European
420 Zone" and for "Mitteleurop��ische Zeit" in German). Nowadays
421 'CET' ("Central European Time") is more common in English, and
422 the database uses 'CET' even for circa-1910 timestamps as this
423 is less confusing for modern users and avoids the need for
424 determining when 'CET' supplanted 'MEZ' in common usage.
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.
425 </li>
426 <li>
476 </li>
477 <li>
427 Use a consistent style in a zone's history. For example, if a zone's
428 history tends to use numeric abbreviations and a particular
429 entry could go either way, use a numeric abbreviation.
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.
430 </li>
431 <li>
482 </li>
483 <li>
432 Use UT (with time zone abbreviation '<code>-</code>00') for
433 locations while uninhabited. The leading
434 '<code>-</code>' is a flag that the time
435 zone is in some sense undefined; this notation is
436 derived from Internet RFC 3339.
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>.
437 </li>
438</ul>
492 </li>
493</ul>
494
439<p>
440Application writers should note that these abbreviations are ambiguous
441in practice: e.g., 'CST' means one thing in China and something else
442in North America, and 'IST' can refer to time in India, Ireland or
495<p>
496Application 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
443Israel. To avoid ambiguity, use numeric UT offsets like
499Israel.
500To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
444'<code>-</code>0600' instead of time zone abbreviations like 'CST'.
445</p>
501'<code>-</code>0600' instead of time zone abbreviations like 'CST'.
502</p>
446 </section>
503
447
504
448
449 <section>
450 <h2 id="accuracy">Accuracy of the tz database</h2>
505<section>
506 <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
451<p>
507<p>
452The tz database is not authoritative, and it surely has errors.
508The <code><abbr>tz</abbr></code> database is not authoritative, and it
509surely has errors.
453Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
454Users requiring authoritative data should consult national standards
455bodies and the references cited in the database's comments.
456</p>
457
458<p>
510Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
511Users requiring authoritative data should consult national standards
512bodies and the references cited in the database's comments.
513</p>
514
515<p>
459Errors in the tz database arise from many sources:
516Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
460</p>
517</p>
518
461<ul>
462 <li>
519<ul>
520 <li>
463 The tz database predicts future timestamps, and current predictions
464 will be incorrect after future governments change the rules.
465 For example, if today someone schedules a meeting for 13:00 next
466 October 1, Casablanca time, and tomorrow Morocco changes its
467 daylight saving rules, software can mess up after the rule change
468 if it blithely relies on conversions made before the change.
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.
469 </li>
470 <li>
528 </li>
529 <li>
471 The pre-1970 entries in this database cover only a tiny sliver of how
472 clocks actually behaved; the vast majority of the necessary
473 information was lost or never recorded. Thousands more zones would
474 be needed if the tz database's scope were extended to cover even
475 just the known or guessed history of standard time; for example,
476 the current single entry for France would need to split into dozens
477 of entries, perhaps hundreds. And in most of the world even this
478 approach would be misleading due to widespread disagreement or
479 indifference about what times should be observed. In her 2015 book
480 <cite>The Global Transformation of Time, 1870-1950</cite>, Vanessa Ogle writes
481 "Outside of Europe and North America there was no system of time
482 zones at all, often not even a stable landscape of mean times,
483 prior to the middle decades of the twentieth century". See:
484 Timothy Shenk, <a
485 href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
486 A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
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.
487 </li>
488 <li>
552 </li>
553 <li>
489 Most of the pre-1970 data entries come from unreliable sources, often
490 astrology books that lack citations and whose compilers evidently
491 invented entries when the true facts were unknown, without
492 reporting which entries were known and which were invented.
493 These books often contradict each other or give implausible entries,
494 and on the rare occasions when they are checked they are
495 typically found to be incorrect.
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.
496 </li>
497 <li>
561 </li>
562 <li>
498 For the UK the tz database relies on years of first-class work done by
499 Joseph Myers and others; see
500 "<a href="https://www.polyomino.org.uk/british-time/">History of
501 legal time in Britain</a>".
502 Other countries are not done nearly as well.
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.
503 </li>
504 <li>
569 </li>
570 <li>
505 Sometimes, different people in the same city would maintain clocks
506 that differed significantly. Railway time was used by railroad
507 companies (which did not always agree with each other),
508 church-clock time was used for birth certificates, etc.
509 Often this was merely common practice, but sometimes it was set by law.
510 For example, from 1891 to 1911 the UT offset in France was legally
511 0:09:21 outside train stations and 0:04:21 inside.
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.
512 </li>
513 <li>
585 </li>
586 <li>
514 Although a named location in the tz database stands for the
515 containing region, its pre-1970 data entries are often accurate for
516 only a small subset of that region. For example, <code>Europe/London</code>
517 stands for the United Kingdom, but its pre-1847 times are valid
518 only for locations that have London's exact meridian, and its 1847
519 transition to GMT is known to be valid only for the L&amp;NW and the
520 Caledonian railways.
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.
521 </li>
522 <li>
595 </li>
596 <li>
523 The tz database does not record the earliest time for which a zone's
524 data entries are thereafter valid for every location in the region.
525 For example, <code>Europe/London</code> is valid for all locations in its
526 region after GMT was made the standard time, but the date of
527 standardization (1880-08-02) is not in the tz database, other than
528 in commentary. For many zones the earliest time of validity is
529 unknown.
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.
530 </li>
531 <li>
606 </li>
607 <li>
532 The tz database does not record a region's boundaries, and in many
533 cases the boundaries are not known. For example, the zone
534 <code>America/Kentucky/Louisville</code> represents a region around
535 the city of
536 Louisville, the boundaries of which are unclear.
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.
537 </li>
538 <li>
614 </li>
615 <li>
539 Changes that are modeled as instantaneous transitions in the tz
540 database were often spread out over hours, days, or even decades.
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.
541 </li>
542 <li>
619 </li>
620 <li>
543 Even if the time is specified by law, locations sometimes
544 deliberately flout the law.
621 Even if the time is specified by law, locations sometimes
622 deliberately flout the law.
545 </li>
546 <li>
623 </li>
624 <li>
547 Early timekeeping practices, even assuming perfect clocks, were
548 often not specified to the accuracy that the tz database requires.
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.
549 </li>
550 <li>
628 </li>
629 <li>
551 Sometimes historical timekeeping was specified more precisely
552 than what the tz database can handle. For example, from 1909 to
553 1937 Netherlands clocks were legally UT +00:19:32.13, but the tz
554 database cannot represent the fractional second.
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.
555 </li>
556 <li>
640 </li>
641 <li>
557 Even when all the timestamp transitions recorded by the tz database
558 are correct, the tz rules that generate them may not faithfully
559 reflect the historical rules. For example, from 1922 until World
560 War II the UK moved clocks forward the day following the third
561 Saturday in April unless that was Easter, in which case it moved
562 clocks forward the previous Sunday. Because the tz database has no
563 way to specify Easter, these exceptional years are entered as
564 separate tz Rule lines, even though the legal rules did not change.
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.
565 </li>
566 <li>
654 </li>
655 <li>
567 The tz database models pre-standard time using the proleptic Gregorian
568 calendar and local mean time (LMT), but many people used other
569 calendars and other timescales. For example, the Roman Empire used
570 the Julian calendar, and had 12 varying-length daytime hours with a
571 non-hour-based system at night.
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.
572 </li>
573 <li>
667 </li>
668 <li>
574 Early clocks were less reliable, and data entries do not represent
575 clock error.
669 Early clocks were less reliable, and data entries do not represent
670 clock error.
576 </li>
577 <li>
671 </li>
672 <li>
578 The tz database assumes Universal Time (UT) as an origin, even
579 though UT is not standardized for older timestamps. In the tz
580 database commentary, UT denotes a family of time standards that
581 includes Coordinated Universal Time (UTC) along with other variants
582 such as UT1 and GMT, with days starting at midnight. Although UT
583 equals UTC for modern timestamps, UTC was not defined until 1960,
584 so commentary uses the more-general abbreviation UT for timestamps
585 that might predate 1960. Since UT, UT1, etc. disagree slightly,
586 and since pre-1972 UTC seconds varied in length, interpretation of
587 older timestamps can be problematic when subsecond accuracy is
588 needed.
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.
589 </li>
590 <li>
689 </li>
690 <li>
591 Civil time was not based on atomic time before 1972, and we don't
592 know the history of earth's rotation accurately enough to map SI
593 seconds to historical solar time to more than about one-hour
594 accuracy. See: Stephenson FR, Morrison LV, Hohenkerk CY.
595 <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement
596 of the Earth's rotation: 720 BC to AD 2015</a>.
597 <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404.
598 Also see: Espenak F. <a
599 href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
600 in Delta T (��T)</a>.
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>.
601 </li>
602 <li>
706 </li>
707 <li>
603 The relationship between POSIX time (that is, UTC but ignoring leap
604 seconds) and UTC is not agreed upon after 1972. Although the POSIX
605 clock officially stops during an inserted leap second, at least one
606 proposed standard has it jumping back a second instead; and in
607 practice POSIX clocks more typically either progress glacially during
608 a leap second, or are slightly slowed while near a leap second.
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.
609 </li>
610 <li>
716 </li>
717 <li>
611 The tz database does not represent how uncertain its information is.
612 Ideally it would contain information about when data entries are
613 incomplete or dicey. Partial temporal knowledge is a field of
614 active research, though, and it's not clear how to apply it here.
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.
615 </li>
616</ul>
724 </li>
725</ul>
617<p>
618In short, many, perhaps most, of the tz database's pre-1970 and future
619timestamps are either wrong or misleading. Any attempt to pass the
620tz database off as the definition of time should be unacceptable to
621anybody who cares about the facts. In particular, the tz database's
622LMT offsets should not be considered meaningful, and should not prompt
623creation of zones merely because two locations differ in LMT or
624transitioned to standard time at different dates.
625</p>
626 </section>
627
726
628
629 <section>
630 <h2 id="functions">Time and date functions</h2>
631<p>
727<p>
632The tz code contains time and date functions that are upwards
633compatible with those of POSIX.
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.
634</p>
740</p>
741</section>
635
742
743<section>
744 <h2 id="functions">Time and date functions</h2>
636<p>
745<p>
637POSIX has the following properties and limitations.
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>.
638</p>
756</p>
757
758<h3 id="POSIX">POSIX properties and limitations</h3>
639<ul>
640 <li>
641 <p>
759<ul>
760 <li>
761 <p>
642 In POSIX, time display in a process is controlled by the
643 environment variable TZ. Unfortunately, the POSIX TZ string takes
644 a form that is hard to describe and is error-prone in practice.
645 Also, POSIX TZ strings can't deal with other (for example, Israeli)
646 daylight saving time rules, or situations where more than two
647 time zone abbreviations are used in an area.
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.
648 </p>
771 </p>
772
649 <p>
773 <p>
650 The POSIX TZ string takes the following form:
774 The POSIX <code>TZ</code> string takes the following form:
651 </p>
775 </p>
776
652 <p>
777 <p>
653 <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>]]]
778 stdoffset[dst[offset][,date[/time],date[/time]]]
654 </p>
779 </p>
780
655 <p>
781 <p>
656 where:
782 where:
783 </p>
784
657 <dl>
658 <dt><var>std</var> and <var>dst</var></dt><dd>
785 <dl>
786 <dt><var>std</var> and <var>dst</var></dt><dd>
659 are 3 or more characters specifying the standard
660 and daylight saving time (DST) zone names.
661 Starting with POSIX.1-2001, <var>std</var>
662 and <var>dst</var> may also be
663 in a quoted form like '<code>&lt;+09&gt;</code>'; this allows
664 "<code>+</code>" and "<code>-</code>" in the names.
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.
665 </dd>
666 <dt><var>offset</var></dt><dd>
792 </dd>
793 <dt><var>offset</var></dt><dd>
667 is of the form
668 '<code>[&plusmn;]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>'
669 and specifies the offset west of UT. '<var>hh</var>'
670 may be a single digit; 0&le;<var>hh</var>&le;24.
671 The default DST offset is one hour ahead of standard time.
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.
672 </dd>
673 <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
801 </dd>
802 <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
674 specifies the beginning and end of DST. If this is absent,
675 the system supplies its own rules for DST, and these can
676 differ from year to year; typically US DST rules are used.
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.
677 </dd>
678 <dt><var>time</var></dt><dd>
807 </dd>
808 <dt><var>time</var></dt><dd>
679 takes the form
680 '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]'
681 and defaults to 02:00.
682 This is the same format as the offset, except that a
683 leading '<code>+</code>' or '<code>-</code>' is not allowed.
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.
684 </dd>
685 <dt><var>date</var></dt><dd>
814 </dd>
815 <dt><var>date</var></dt><dd>
686 takes one of the following forms:
816 takes one of the following forms:
687 <dl>
688 <dt>J<var>n</var> (1&le;<var>n</var>&le;365)</dt><dd>
817 <dl>
818 <dt>J<var>n</var> (1&le;<var>n</var>&le;365)</dt><dd>
689 origin-1 day number not counting February 29
690 </dd>
819 origin-1 day number not counting February 29
820 </dd>
691 <dt><var>n</var> (0&le;<var>n</var>&le;365)</dt><dd>
821 <dt><var>n</var> (0&le;<var>n</var>&le;365)</dt><dd>
692 origin-0 day number counting February 29 if present
693 </dd>
694 <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>
695 for the <var>d</var>th day of
696 week <var>n</var> of month <var>m</var> of the
697 year, where week 1 is the first week in which
698 day <var>d</var> appears, and '<code>5</code>'
699 stands for the last week in which
700 day <var>d</var> appears
701 (which may be either the 4th or 5th week).
702 Typically, this is the only useful form;
703 the <var>n</var>
704 and <code>J</code><var>n</var> forms are
705 rarely used.
822 origin-0 day number counting February 29 if present
706 </dd>
823 </dd>
707</dl>
708</dd>
709</dl>
710 Here is an example POSIX TZ string for New Zealand after 2007.
711 It says that standard time (NZST) is 12 hours ahead of UT,
712 and that daylight saving time (NZDT) is observed from September's
713 last Sunday at 02:00 until April's first Sunday at 03:00:
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>
714
839
715 <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
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>
716
848
717 This POSIX TZ string is hard to remember, and mishandles some
718 timestamps before 2008. With this package you can use this
719 instead:
849 <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
720
850
721 <pre><code>TZ='Pacific/Auckland'</code></pre>
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>
722 </li>
723 <li>
858 </li>
859 <li>
724 POSIX does not define the exact meaning of TZ values like
725 "<code>EST5EDT</code>".
726 Typically the current US DST rules are used to interpret such values,
727 but this means that the US DST rules are compiled into each program
728 that does time conversion. This means that when US time conversion
729 rules change (as in the United States in 1987), all programs that
730 do time conversion must be recompiled to ensure proper results.
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.
731 </li>
732 <li>
870 </li>
871 <li>
733 The TZ environment variable is process-global, which makes it hard
734 to write efficient, thread-safe applications that need access
735 to multiple time zones.
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.
736 </li>
737 <li>
875 </li>
876 <li>
738 In POSIX, there's no tamper-proof way for a process to learn the
739 system's best idea of local wall clock. (This is important for
740 applications that an administrator wants used only at certain
741 times &ndash;
742 without regard to whether the user has fiddled the TZ environment
743 variable. While an administrator can "do everything in UT" to get
744 around the problem, doing so is inconvenient and precludes handling
745 daylight saving time shifts - as might be required to limit phone
746 calls to off-peak hours.)
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.)
747 </li>
748 <li>
887 </li>
888 <li>
749 POSIX provides no convenient and efficient way to determine the UT
750 offset and time zone abbreviation of arbitrary timestamps,
751 particularly for time zone settings that do not fit into the
752 POSIX model.
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.
753 </li>
754 <li>
893 </li>
894 <li>
755 POSIX requires that systems ignore leap seconds.
895 POSIX requires that systems ignore leap seconds.
756 </li>
757 <li>
896 </li>
897 <li>
758 The tz code attempts to support all the <code>time_t</code>
759 implementations allowed by POSIX. The <code>time_t</code>
760 type represents a nonnegative count of
761 seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds.
762 In practice, <code>time_t</code> is usually a signed 64- or
763 32-bit integer; 32-bit signed <code>time_t</code> values stop
764 working after 2038-01-19 03:14:07 UTC, so
765 new implementations these days typically use a signed 64-bit integer.
766 Unsigned 32-bit integers are used on one or two platforms,
767 and 36-bit and 40-bit integers are also used occasionally.
768 Although earlier POSIX versions allowed <code>time_t</code> to be a
769 floating-point type, this was not supported by any practical
770 systems, and POSIX.1-2013 and the tz code both
771 require <code>time_t</code>
772 to be an integer type.
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.
773 </li>
774</ul>
912 </li>
913</ul>
775<p>
776These are the extensions that have been made to the POSIX functions:
777</p>
914
915<h3 id="POSIX-extensions">Extensions to POSIX in the
916<code><abbr>tz</abbr></code> code</h3>
778<ul>
779 <li>
780 <p>
917<ul>
918 <li>
919 <p>
781 The TZ environment variable is used in generating the name of a file
782 from which time zone information is read (or is interpreted a la
783 POSIX); TZ is no longer constrained to be a three-letter time zone
784 name followed by a number of hours and an optional three-letter
785 daylight time zone name. The daylight saving time rules to be used
786 for a particular time zone are encoded in the time zone file;
787 the format of the file allows U.S., Australian, and other rules to be
788 encoded, and allows for situations where more than two time zone
789 abbreviations are used.
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.
790 </p>
791 <p>
932 </p>
933 <p>
792 It was recognized that allowing the TZ environment variable to
793 take on values such as '<code>America/New_York</code>' might
794 cause "old" programs
795 (that expect TZ to have a certain form) to operate incorrectly;
796 consideration was given to using some other environment variable
797 (for example, TIMEZONE) to hold the string used to generate the
798 time zone information file name. In the end, however, it was decided
799 to continue using TZ: it is widely used for time zone purposes;
800 separately maintaining both TZ and TIMEZONE seemed a nuisance;
801 and systems where "new" forms of TZ might cause problems can simply
802 use TZ values such as "<code>EST5EDT</code>" which can be used both by
803 "new" programs (a la POSIX) and "old" programs (as zone names and
804 offsets).
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).
805 </p>
948 </p>
806
807

  • 808 The code supports platforms with a UT offset member
    809 in <code>struct tm</code>,
    810 e.g., <code>tm_gmtoff</code>.
    811</li>
    812<li>
    813 The code supports platforms with a time zone abbreviation member in
    814 <code>struct tm</code>, e.g., <code>tm_zone</code>.
    815</li>
    816<li>
    817 Since the TZ environment variable can now be used to control time
    818 conversion, the <code>daylight</code>
    819 and <code>timezone</code> variables are no longer needed.
    820 (These variables are defined and set by <code>tzset</code>;
    821 however, their values will not be used
    822 by <code>localtime</code>.)
    823</li>
    824<li>
    825 Functions <code>tzalloc</code>, <code>tzfree</code>,
    826 <code>localtime_rz</code>, and <code>mktime_z</code> for
    827 more-efficient thread-safe applications that need to use
    828 multiple time zones. The <code>tzalloc</code>
    829 and <code>tzfree</code> functions allocate and free objects of
    830 type <code>timezone_t</code>, and <code>localtime_rz</code>
    831 and <code>mktime_z</code> are like <code>localtime_r</code>
    832 and <code>mktime</code> with an extra
    833 <code>timezone_t</code> argument. The functions were inspired
    834 by NetBSD.
    835</li>
    836<li>
    837 A function <code>tzsetwall</code> has been added to arrange
    838 for the system's
    839 best approximation to local wall clock time to be delivered by
    840 subsequent calls to <code>localtime</code>. Source code for portable
    841 applications that "must" run on local wall clock time should call
    842 <code>tzsetwall</code>; if such code is moved to "old" systems that don't
    843 provide tzsetwall, you won't be able to generate an executable program.
    844 (These time zone functions also arrange for local wall clock time to be
    845 used if tzset is called &ndash; directly or indirectly &ndash;
    846 and there's no TZ
    847 environment variable; portable applications should not, however, rely
    848 on this behavior since it's not the way SVR2 systems behave.)
    849</li>
    850<li>
    851 Negative <code>time_t</code> values are supported, on systems
    852 where <code>time_t</code> is signed.
    853</li>
    854<li>
    855 These functions can account for leap seconds, thanks to Bradley White.
    856</li>
    		• 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>
    857</ul>
    		992</ul>
    993
    994<h3 id="vestigial">POSIX features no longer needed</h3>
    858<p>
    		995<p>
    859Points of interest to folks with other systems:
    		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:
    860</p>
    861<ul>
    862 <li>
    		1005</p>
    1006<ul>
    1007 <li>
    863 Code compatible with this package is already part of many platforms,
    864 including GNU/Linux, Android, the BSDs, Chromium OS, Cygwin, AIX, iOS,
    865 BlackBery 10, macOS, Microsoft Windows, OpenVMS, and Solaris.
    866 On such hosts, the primary use of this package
    867 is to update obsolete time zone rule tables.
    868 To do this, you may need to compile the time zone compiler
    869 '<code>zic</code>' supplied with this package instead of using
    870 the system '<code>zic</code>', since the format
    871 of <code>zic</code>'s input is occasionally extended, and a
    872 platform may still be shipping an older <code>zic</code>.
    		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.
    873 </li>
    874 <li>
    		1014 </li>
    1015 <li>
    875 The UNIX Version 7 <code>timezone</code> function is not
    876 present in this package;
    877 it's impossible to reliably map timezone's arguments (a "minutes west
    878 of GMT" value and a "daylight saving time in effect" flag) to a
    879 time zone abbreviation, and we refuse to guess.
    880 Programs that in the past used the timezone function may now examine
    881 <code>localtime(&amp;clock)-&gt;tm_zone</code>
    882 (if <code>TM_ZONE</code> is defined) or
    883 <code>tzname[localtime(&amp;clock)-&gt;tm_isdst]</code>
    884 (if <code>HAVE_TZNAME</code> is defined)
    885 to learn the correct time zone abbreviation to use.
    		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.
    886 </li>
    887 <li>
    		1024 </li>
    1025 <li>
    888 The 4.2BSD <code>gettimeofday</code> function is not used in
    889 this package.
    890 This formerly let users obtain the current UTC offset and DST flag,
    891 but this functionality was removed in later versions of BSD.
    		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.
    892 </li>
    		1035 </li>
    1036</ul>
    1037
    1038<h3 id="other-portability">Other portability notes</h3>
    1039<ul>
    893 <li>
    		1040 <li>
    894 In SVR2, time conversion fails for near-minimum or near-maximum
    895 <code>time_t</code> values when doing conversions for places
    896 that don't use UT.
    897 This package takes care to do these conversions correctly.
    898 A comment in the source code tells how to get compatibly wrong
    899 results.
    		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.
    900 </li>
    		1053 </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>
    901</ul>
    		1091</ul>
    902<p>
    903The functions that are conditionally compiled
    904if <code>STD_INSPIRED</code> is defined
    905should, at this point, be looked on primarily as food for thought. They are
    906not in any sense "standard compatible" &ndash; some are not, in fact,
    907specified in <em>any</em> standard. They do, however, represent responses of
    908various authors to
    909standardization proposals.
    910</p>
    		1092</section>
    911
    		1093
    1094<section>
    1095 <h2 id="stability">Interface stability</h2>
    912<p>
    		1096<p>
    913Other time conversion proposals, in particular the one developed by folks at
    914Hewlett Packard, offer a wider selection of functions that provide capabilities
    915beyond those provided here. The absence of such functions from this package
    916is not meant to discourage the development, standardization, or use of such
    917functions. Rather, their absence reflects the decision to make this package
    918contain valid extensions to POSIX, to ensure its broad acceptability. If
    919more powerful time conversion functions can be standardized, so much the
    920better.
    		1097The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
    921</p>
    		1098</p>
    922 </section>
    923
    		1099
    924
    925 <section>
    926 <h2 id="stability">Interface stability</h2>
    927<p>
    928The tz code and data supply the following interfaces:
    929</p>
    930<ul>
    931 <li>
    		1100<ul>
    1101 <li>
    932 A set of zone names as per "<a href="#naming">Names of time zone
    933 rules</a>" above.
    		1102 A set of <code><abbr>tz</abbr></code> region names as per
    1103 "<a href="#naming">Names of time zone rulesets</a>" above.
    934 </li>
    935 <li>
    		1104 </li>
    1105 <li>
    936 Library functions described in "Time and date
    937 functions    " above.
    		1106 Library functions described in "<a href="#functions">Time and date
    1107 functions</a>" above.
    938 </li>
    939 <li>
    		1108 </li>
    1109 <li>
    940 The programs tzselect, zdump,
    941 and zic, documented in their man pages.
    		1110 The programs <code>tzselect</code>, <code>zdump</code>,
    1111 and <code>zic</code>, documented in their man pages.
    942 </li>
    943 <li>
    		1112 </li>
    1113 <li>
    944 The format of zic input files, documented in
    945 the zic man page.
    		1114 The format of <code>zic</code> input files, documented in
    1115 the <code>zic</code> man page.
    946 </li>
    947 <li>
    		1116 </li>
    1117 <li>
    948 The format of zic output files, documented in
    949 the tzfile man page.
    		1118 The format of <code>zic</code> output files, documented in
    1119 the <code>tzfile</code> man page.
    950 </li>
    951 <li>
    		1120 </li>
    1121 <li>
    952 The format of zone table files, documented in zone1970.tab.
    		1122 The format of zone table files, documented in <code>zone1970.tab</code>.
    953 </li>
    954 <li>
    		1123 </li>
    1124 <li>
    955 The format of the country code file, documented in iso3166.tab.
    		1125 The format of the country code file, documented in <code>iso3166.tab</code>.
    956 </li>
    957 <li>
    		1126 </li>
    1127 <li>
    958 The version number of the code and data, as the first line of
    959 the text file 'version' in each release.
    		1128 The version number of the code and data, as the first line of
    1129 the text file '<code>version</code>' in each release.
    960 </li>
    961</ul>
    		1130 </li>
    1131</ul>
    1132
    962<p>
    963Interface changes in a release attempt to preserve compatibility with
    		1133<p>
    1134Interface changes in a release attempt to preserve compatibility with
    964recent releases. For example, tz data files typically do not rely on
    965recently-added <code>zic</code> features, so that users can run
    966older <code>zic</code> versions to process newer data
    967files. <a href="tz-link.html">Sources for time zone and daylight
    968saving time data</a> describes how
    969releases are tagged and distributed.
    		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.
    970</p>
    971
    972<p>
    		1142</p>
    1143
    1144<p>
    973Interfaces not listed above are less stable. For example, users
    974should not rely on particular UT offsets or abbreviations for
    975timestamps, as data entries are often based on guesswork and these
    976guesses may be corrected or improved.
    		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.
    977</p>
    		1149</p>
    978 </section>
    		1150
    979
    		1151
    980
    981 <section>
    982 <h2 id="calendar">Calendrical issues</h2>
    		1152<section>
    1153 <h2 id="calendar">Calendrical issues</h2>
    983<p>
    984Calendrical issues are a bit out of scope for a time zone database,
    985but they indicate the sort of problems that we would run into if we
    		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
    986extended the time zone database further into the past. An excellent
    987resource in this area is Nachum Dershowitz and Edward M. Reingold,
    988<cite><a href="https://www.cs.tau.ac.il/~nachum/calendar-book/third-edition/">Calendrical
    		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
    989Calculations: Third Edition</a></cite>, Cambridge University Press (2008).
    		1161Calculations: Third Edition</a></cite>, Cambridge University Press (2008).
    990Other information and sources are given in the file '<samp>calendars</samp>'
    991in the tz distribution. They sometimes disagree.
    		1162Other information and sources are given in the file '<code>calendars</code>'
    1163in the <code><abbr>tz</abbr></code> distribution.
    1164They sometimes disagree.
    992</p>
    		1165</p>
    993 </section>
    		1166
    994
    		1167
    995
    996 <section>
    997 <h2 id="planets">Time and time zones on other planets</h2>
    		1168<section>
    1169 <h2 id="planets">Time and time zones on other planets</h2>
    998<p>
    		1170<p>
    999Some people's work schedules use Mars time. Jet Propulsion Laboratory
    1000(JPL) coordinators have kept Mars time on and off at least since 1997
    1001for the Mars Pathfinder mission. Some of their family members have
    1002also adapted to Mars time. Dozens of special Mars watches were built
    1003for JPL workers who kept Mars time during the Mars Exploration
    1004Rovers mission (2004). These timepieces look like normal Seikos and
    1005Citizens but use Mars seconds rather than terrestrial seconds.
    		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.
    1006</p>
    1007
    1008<p>
    1009A Mars solar day is called a "sol" and has a mean period equal to
    		1182</p>
    1183
    1184<p>
    1185A Mars solar day is called a "sol" and has a mean period equal to
    1010about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is
    1011divided into a conventional 24-hour clock, so each Mars second equals
    1012about 1.02749125 terrestrial seconds.
    		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.
    1013</p>
    1014
    1015<p>
    		1189</p>
    1190
    1191<p>
    1016The prime meridian of Mars goes through the center of the crater
    1017Airy-0, named in honor of the British astronomer who built the
    1018Greenwich telescope that defines Earth's prime meridian. Mean solar
    1019time on the Mars prime meridian is called Mars Coordinated Time (MTC).
    		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>.
    1020</p>
    1021
    1022<p>
    1023Each landed mission on Mars has adopted a different reference for
    1024solar time keeping, so there is no real standard for Mars time zones.
    		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.
    1025For example, the Mars Exploration Rover project (2004) defined two
    1026time zones "Local Solar Time A" and "Local Solar Time B" for its two
    1027missions, each zone designed so that its time equals local true solar
    1028time at approximately the middle of the nominal mission. Such a "time
    1029zone" is not particularly suited for any application other than the
    1030mission itself.
    		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.
    1031</p>
    1032
    1033<p>
    1034Many calendars have been proposed for Mars, but none have achieved
    		1213</p>
    1214
    1215<p>
    1216Many calendars have been proposed for Mars, but none have achieved
    1035wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a
    		1217wide acceptance.
    1218Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
    1036sequential count of Mars solar days elapsed since about 1873-12-29
    		1219sequential count of Mars solar days elapsed since about 1873-12-29
    103712:00 GMT.
    		122012:00 <abbr>GMT</abbr>.
    1038</p>
    1039
    1040<p>
    1041In our solar system, Mars is the planet with time and calendar most
    		1221</p>
    1222
    1223<p>
    1224In our solar system, Mars is the planet with time and calendar most
    1042like Earth's. On other planets, Sun-based time and calendars would
    1043work quite differently. For example, although Mercury's sidereal
    1044rotation period is 58.646 Earth days, Mercury revolves around the Sun
    1045so rapidly that an observer on Mercury's equator would see a sunrise
    1046only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a Mercury
    1047day. Venus is more complicated, partly because its rotation is
    1048slightly retrograde: its year is 1.92 of its days. Gas giants like
    1049Jupiter are trickier still, as their polar and equatorial regions
    1050rotate at different rates, so that the length of a day depends on
    1051latitude. This effect is most pronounced on Neptune, where the day is
    1052about 12 hours at the poles and 18 hours at the equator.
    		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.
    1053</p>
    1054
    1055<p>
    		1242</p>
    1243
    1244<p>
    1056Although the tz database does not support time on other planets, it is
    1057documented here in the hopes that support will be added eventually.
    		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.
    1058</p>
    1059
    1060<p>
    		1248</p>
    1249
    1250<p>
    1061Sources:
    		1251Sources for time on other planets:
    1062</p>
    		1252</p>
    1253
    1063<ul>
    1064 <li>
    		1254<ul>
    1255 <li>
    1065Michael Allison and Robert Schmunk,
    1066"Technical
    1067Notes on Mars Solar Time as Adopted by the Mars24 Sunclock    "
    1068(2015-06-30).
    		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).
    1069 </li>
    1070 <li>
    		1260 </li>
    1261 <li>
    1071Jia-Rui Chong,
    1072"Workdays
    1073Fit for a Martian</a>", Los Angeles Times
    1074(2004-01-14), pp A1, A20-A21.
    		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.
    1075 </li>
    1076 <li>
    		1266 </li>
    1267 <li>
    1077Tom Chmielewski,
    1078"Jet
    1079Lag Is Worse on Mars</a>", The Atlantic (2015-02-26)
    		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)
    1080 </li>
    1081 <li>
    		1271 </li>
    1272 <li>
    1082Matt Williams,
    1083"How
    1084long is a day on the other planets of the solar system?    "
    1085(2017-04-27).
    		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).
    1086 </li>
    1087</ul>
    		1277 </li>
    1278</ul>
    1088 </section>
    		1279
    1089
    		1280
    1090 <footer>
    1091 <hr>
    1092This file is in the public domain, so clarified as of 2009-05-17 by
    1093Arthur David Olson.
    1094 </footer>
    		1281

    1282

    1283 This file is in the public domain, so clarified as of 2009-05-17 by
    1284 Arthur David Olson.
    1285
    1095</body>
    1096</html>
    		1286</body>
    1287</html>