-
Notifications
You must be signed in to change notification settings - Fork 1
/
tz-link.html
1184 lines (1165 loc) · 57.8 KB
/
tz-link.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html lang="en">
<head>
<title>Sources for time zone and daylight saving time data</title>
<meta charset="UTF-8">
<style>
pre {margin-left: 2em; white-space: pre-wrap;}
</style>
</head>
<body>
<h1>Sources for time zone and daylight saving time data</h1>
<p>
<a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
<a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
rules are controlled by individual
governments. They are sometimes changed with little notice, and their
histories and planned futures are often recorded only fitfully. Here
is a summary of attempts to organize and record relevant data in this
area.
</p>
<h3>Outline</h3>
<nav>
<ul>
<li>The <code><abbr>tz</abbr></code> database product and process
<ul>
<li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
<li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
<li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
<li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
</ul>
</li>
<li>Uses of the <code><abbr>tz</abbr></code> database
<ul>
<li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
<li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
<li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
<li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
<li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
</ul>
</li>
<li>Related data
<ul>
<li><a href="#other-dbs">Other time zone databases</a></li>
<li><a href="#maps">Maps</a></li>
<li><a href="#boundaries">Time zone boundaries</a></li>
</ul>
</li>
<li>Timekeeping concepts
<ul>
<li><a href="#civil">Civil time concepts and history</a></li>
<li><a href="#national">National histories of legal time</a></li>
<li><a href="#costs">Costs and benefits of time shifts</a></li>
<li><a href="#precision">Precision timekeeping</a></li>
<li><a href="#notation">Time notation</a></li>
<li><a href="#see-also">See also</a></li>
</ul>
</li>
</ul>
</nav>
<section>
<h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
<p>
The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
time zone database contains code and data
that represent the history of local time
for many representative locations around the globe.
It is updated periodically to reflect changes made by political bodies
to time zone boundaries and daylight saving rules.
This database (known as <code><abbr>tz</abbr></code>,
<code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
is used by several implementations,
including
<a href="https://www.gnu.org/software/libc/">the
<abbr title="GNU's Not Unix">GNU</abbr>
C Library</a> (used in
<a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
<a href="https://www.android.com">Android</a>,
<a href="https://www.freebsd.org">Free<abbr
title="Berkeley Software Distribution">BSD</abbr></a>,
<a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
<a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
<a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
<a href="https://cygwin.com">Cygwin</a>,
<a href="https://mariadb.org">MariaDB</a>,
<a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
<a href="https://www.mysql.com">MySQL</a>,
<a href="https://en.wikipedia.org/wiki/WebOS"><abbr
title="Web Operating System">webOS</abbr></a>,
<a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
title="Advanced Interactive eXecutive">AIX</abbr></a>,
<a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
<a href="https://www.apple.com/ios"><abbr
title="iPhone OS">iOS</abbr></a>,
<a href="https://www.apple.com/macos">macOS</a>,
<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
<a href="https://www.vmssoftware.com">Open<abbr
title="Virtual Memory System">VMS</abbr></a>,
<a href="https://www.oracle.com/database/">Oracle Database</a>, and
<a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
<p>
Each main entry in the database represents a <dfn>timezone</dfn>
for a set of civil-time clocks that have all agreed since 1970.
Timezones are typically identified by continent or ocean and then by the
name of the largest city within the region containing the clocks.
For example, <code>America/New_York</code>
represents most of the <abbr title="United States">US</abbr> eastern time zone;
<code>America/Phoenix</code> represents most of Arizona, which
uses mountain time without daylight saving time (<abbr>DST</abbr>);
<code>America/Detroit</code> represents most of Michigan, which uses
eastern time but with different <abbr>DST</abbr> rules in 1975;
and other entries represent smaller regions like Starke County,
Indiana, which switched from central to eastern time in 1991
and switched back in 2006.
To use the database on an extended <a
href="https://en.wikipedia.org/wiki/POSIX"><abbr
title="Portable Operating System Interface">POSIX</abbr></a>
implementation set the <code><abbr>TZ</abbr></code>
environment variable to the location's full name,
e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
<p>
Associated with each timezone is a history of offsets from
<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
Time</a> (<abbr>UT</abbr>), which is <a
href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
for timestamps after 1960 this is more precisely <a
href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
Universal Time</a> (<abbr>UTC</abbr>).
The database also records when daylight saving time was in use,
along with some time zone abbreviations such as <abbr>EST</abbr>
for Eastern Standard Time in the <abbr>US</abbr>.</p>
</section>
<section>
<h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
<p>
The following <a
href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
the latest release's two
<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
to a <abbr>GNU</abbr>/Linux or similar host.</p>
<pre><code>mkdir tzdb
cd tzdb
<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
<a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
gzip -dc tzdata-latest.tar.gz | tar -xf -
</code></pre>
<p>Alternatively, the following shell commands download the same
release in a single-tarball format containing extra data
useful for regression testing:</p>
<pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
<a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
</code></pre>
<p>These commands use convenience links to the latest release
of the <code><abbr>tz</abbr></code> database hosted by the
<a href="https://www.iana.org/time-zones">Time Zone Database website</a>
of the <a href="https://www.iana.org">Internet Assigned Numbers
Authority (IANA)</a>.
Older releases are in files named
<code>tzcode<var>V</var>.tar.gz</code>,
<code>tzdata<var>V</var>.tar.gz</code>, and
<code>tzdb-<var>V</var>.tar.lz</code>,
where <code><var>V</var></code> is the version.
Since 1996, each version has been a four-digit year followed by
lower-case letter (<samp>a</samp> through <samp>z</samp>,
then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
through <samp>zzz</samp>, and so on).
Since version 1999g, each release has been distributed in
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
ustar interchange format</a>, compressed as described above;
older releases use a nearly-compatible format.
Since version 2016h, each release has contained a text file named
"<samp>version</samp>" whose first (and currently only) line is the version.
Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
and are also available in an
<a href="ftp://ftp.iana.org/tz/releases/"><abbr
title="File Transfer Protocol">FTP</abbr> directory</a> via a
less-secure protocol.</p>
<p>Alternatively, a development repository of code and data can be
retrieved from <a href="https://github.com">GitHub</a> via the shell
command:</p>
<pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
</code></pre>
<p>
Since version 2012e, each release has been tagged in development repositories.
Untagged commits are less well tested and probably contain
more errors.</p>
<p>
After obtaining the code and data files, see the
<code>README</code> file for what to do next.
The code lets you compile the <code><abbr>tz</abbr></code> source files into
machine-readable binary files, one for each location. The binary files
are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
specified by <a href="https://tools.ietf.org/html/8536">Internet
<abbr>RFC</abbr> 8536</a>.
The code also lets
you read a <abbr>TZif</abbr> file and interpret timestamps for that
location.</p>
</section>
<section>
<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
<p>
The <code><abbr>tz</abbr></code> code and data
are by no means authoritative. If you find errors, please
send changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>,
the time zone mailing list. You can also <a
href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
and browse the <a
href="https://mm.icann.org/pipermail/tz/">archive of old
messages</a>.
<a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
discussions</a> and corresponding data changes can be
generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
</p>
<p>
If your government plans to change its time zone boundaries or
daylight saving rules, inform <code>tz@iana.org</code> well in
advance, as this will coordinate updates to many cell phones,
computers, and other devices around the world.
The change should be officially announced at least a year before it affects
how clocks operate; otherwise, there is a good chance that some
clocks will operate incorrectly after the change, due
to delays in propagating updates to software and data. The shorter
the notice, the more likely clock problems will arise; see "<a
href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
the Timing of Time Zone Changes</a>" for examples.
The <code><abbr>tz</abbr></code> data can represent planned changes
far into the future, and a long-planned change can easily be reverted
or otherwise altered with a year's notice before the change would have
affected clocks.
</p>
<p>
Changes to the <code><abbr>tz</abbr></code> code and data are often
propagated to clients via operating system updates, so
client <code><abbr>tz</abbr></code> data can often be corrected by
applying these updates. With GNU/Linux and similar systems, if your
maintenance provider has not yet adopted the
latest <code><abbr>tz</abbr></code> data, you can often short-circuit
the process by tailoring the generic instructions in
the <code><abbr>tz</abbr> README</code> file and installing the latest
data yourself. System-specific instructions for installing the
latest <code><abbr>tz</abbr></code> data have also been published
for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
<a
href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
<a
href="http://userguide.icu-project.org/datetime/timezone"><abbr
title="International Components for Unicode">ICU</abbr></a>,
<a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr>
JDK</a>,
<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html#usage">OpenJDK/Oracle JDK</a>.
</p>
<p>Sources for the <code><abbr>tz</abbr></code> database are
<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
<a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
title="linefeed">LF</abbr></a>,
which can be modified by common text editors such
as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
<a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
<a href="https://www.vim.org">vim</a>.
Specialized source-file editing can be done via the
<a href="https://packagecontrol.io/packages/zoneinfo">Sublime
zoneinfo</a> package for <a
href="https://www.sublimetext.com">Sublime Text</a> and the <a
href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
Studio Code</a>.
</p>
<p>
For further information about updates, please see
<a href="https://tools.ietf.org/html/rfc6557">Procedures for
Maintaining the Time Zone Database</a> (Internet <abbr
title="Request For Comments">RFC</abbr> 6557). More detail can be
found in <a href="theory.html">Theory and pragmatics of the
<code><abbr>tz</abbr></code> code and data</a>.
<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
displays changes between recent <code><abbr>tzdb</abbr></code> versions.
</p>
</section>
<section>
<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
<ul>
<li>The article
<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
an encyclopedic summary.</li>
<li><a href="tz-how-to.html">How to Read the
tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
database format.</li>
<li><a
href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
database's style.</li>
<li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
managing time in the internet</a> analyzes the database longitudinally.</li>
</ul>
</section>
<section>
<h2 id="web">Web sites using recent versions of the
<code><abbr>tz</abbr></code> database</h2>
<p>
These are listed roughly in ascending order of complexity and fanciness.
</p>
<ul>
<li><a href="https://time.is">Time.is</a> shows locations'
time and zones.</li>
<li><a href="https://www.timejones.com">TimeJones.com</a>,
<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
<a href="https://www.worldclock.com">The World Clock</a>
are time zone converters.</li>
<li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
publishes <code><abbr>tzdb</abbr></code>-derived data in
<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
title="comma-separated values">CSV</abbr></a> and
in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
title="Structured Query Language">SQL</abbr></a> form.</li>
<li><a
href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
<li><a
href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
Time in 1000 Places</a> uses descriptions of the values.</li>
<li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
uses a pulldown menu.</li>
<li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
timezone information for all countries</a>
displays tables of <abbr>DST</abbr> rules.
<li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
Worldwide</a> lets you sort zone names and convert times.</li>
<li><a href="https://24timezones.com">24TimeZones</a> has a world
time map and a time converter.</li>
<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
calculates the current time difference between locations.</li>
<li><a href="http://www.wx-now.com">Weather Now</a> and
<a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
</ul>
</section>
<section>
<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
<ul>
<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
Distribution Service (tzdist) working group</a> defined <a
href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
transferring time zone data by reference.
<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
implementations</a> are available.
The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
mailing list</a> discusses possible extensions.</li>
<li>The <a href="https://tools.ietf.org/html/rfc5545">
Internet Calendaring and Scheduling Core Object Specification
(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
covers time zone
data; see its VTIMEZONE calendar component.
The iCalendar format requires specialized parsers and generators; a
variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
(Internet <abbr>RFC</abbr> 6321) uses
<a href="https://www.w3.org/XML/"><abbr
title="Extensible Markup Language">XML</abbr></a> format, and a variant
<a href="https://tools.ietf.org/html/rfc7265">jCal</a>
(Internet <abbr>RFC</abbr> 7265)
uses <a href="https://www.json.org"><abbr
title="JavaScript Object Notation">JSON</abbr></a> format.</li>
</ul>
</section>
<section>
<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
<p>Although some of these do not fully support
<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
distributions you can generally work around compatibility problems by
running the command <code>make rearguard_tarballs</code> and compiling
from the resulting tarballs instead.</p>
<ul>
<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
program that compiles
<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
Vzic is freely
available under the <a
href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
General Public License (<abbr
title="General Public License">GPL</abbr>)</a>.</li>
<li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
database conversion utility</a> is like Vzic, except for the <a
href="https://dotnet.microsoft.com">.NET framework</a>
and with a <abbr>BSD</abbr>-style license.</li>
<li><a
href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
contains a script <code>parse_olson</code> that compiles
<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
modules. It is part of the Perl <a
href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
which is freely
available under both the <abbr>GPL</abbr> and the Perl Artistic
License. DateTime::TimeZone also contains a script
<code>tests_from_zdump</code> that generates test cases for each clock
transition in the <code><abbr>tz</abbr></code> database.</li>
<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
Database Parser</a> is a
<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
runtime library with <a
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
adopted by
<a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
the current iteration of the C++ standard.
It is freely available under the
<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
<li><a id="ICU" href="http://site.icu-project.org">International Components for
Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
libraries for internationalization that
has a compiler from <code><abbr>tz</abbr></code> source
and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
(mentioned <a href="#CLDR">below</a>)
into an <abbr>ICU</abbr>-specific format.
<abbr>ICU</abbr> is freely available under a
<abbr>BSD</abbr>-style license.</li>
<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
the <a href="https://elixir-lang.org">Elixir</a> language downloads
and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
title="Application Program Interface">API</abbr>s for use. It is
freely available under the <abbr>MIT</abbr> license.</li>
<li>Java-based compilers and libraries include:
<ul>
<li>The <a
href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
<a href="https://openjdk.java.net/">OpenJDK</a> and
<a href="https://jdk.java.net/">Oracle JDK</a>.
Although its source code is proprietary, its executable is available under the
<a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
Timezone Updater License Agreement</a>.</li>
<li>The <a
href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
which is freely available under a <abbr>BSD</abbr>-style license.</li>
<li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
and time <abbr>API</abbr></a> contains a class
<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
<code><abbr>tz</abbr></code> source into a binary format. It inspired
Java 8 <code>java.time</code>, which its users should migrate to once
they can assume Java 8 or later. It is available under the <a
href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
<li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
are alternatives to TZUpdater. IANA Updater's license is unclear;
ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
<li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
time library for Android</a> and
<a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
time and interval library for Java</a> compile
<code><abbr>tz</abbr></code> source into a binary format.
Time4A is available under the Apache License and Time4J is
available under the <a
href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
General Public License (<abbr title="Lesser General Public
License">LGPL</abbr>)</a>.</li>
<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
Java-based libraries.</li>
</ul>
<li><a href="https://nodatime.org">Noda Time – Date and
time <abbr>API</abbr> for .NET</a>
is like Joda-Time and Time4J, but for the .NET framework instead of Java.
It is freely available under the Apache License.</li>
<li>Many modern
<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
runtimes support <code><abbr>tz</abbr></code> natively via the
<samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
This can be used as-is or with most of the following libraries,
many of which also support runtimes lacking the <samp>timeZone</samp> option.
<ul>
<li>The <a
href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
timezone polyfill</a>
is freely available under a <abbr>BSD</abbr>-style license.</li>
<li><a
href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
compiles time zone data into a compact form designed for
JavaScript. It is freely available under a combination of
the <abbr>MIT</abbr> license and the Apache License.</li>
<li>The <a href="https://date-fns.org/">date-fns</a>
library manipulates timezone-aware timestamps in browsers and
in <a href="https://nodejs.org/en/">Node.js</a>.
It is freely available under the <abbr>MIT</abbr> license.</li>
<li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
minimalist replacement for the date and time API of
the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
manipulation library.
It is freely available under the <abbr>MIT</abbr> license.</li>
<li><a href="https://moment.github.io/luxon/">Luxon</a> improves
timezone support for the <samp>Intl</samp> API.
It is freely available under the <abbr>MIT</abbr> license.</li>
<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
Moment.js plugin.
It is freely available under the <abbr>MIT</abbr> license.</li>
<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
JavaScript library that supports date arithmetic that is time zone
aware. It is freely available under the <abbr>MIT</abbr> license.</li>
<li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
supports live <code><abbr>tzdb</abbr></code> updates,
astronomical and atomic time, a command-line interface,
and full <a
href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
Its companion <a
href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
can generate <abbr>TZif</abbr> and other files, and a companion website
<a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
convert timestamps, view transition histories, and download code and data.
It is freely available under the <abbr>MIT</abbr> license.</li>
</ul>
The proposed <a
href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
objects</a> let programs access an abstract view of
<code><abbr>tzdb</abbr></code> data, and are designed to replace <a
href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
problematic <samp>Date</samp> objects</a> when working with dates and times.
<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
compiler from <code><abbr>tz</abbr></code> source into
<a href="https://julialang.org/">Julia</a>. It is freely available
under the <abbr>MIT</abbr> license.</li>
<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
title="Free Pascal Compiler">FPC</abbr></a>
compiles from <code><abbr>tz</abbr></code> source into
<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
and <a
href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
It is freely available under a <abbr>BSD</abbr>-style license.</li>
<li><a href="http://pytz.sourceforge.net">pytz – World Timezone
Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
<a href="https://www.python.org">Python</a>.
It is freely available under a <abbr>BSD</abbr>-style license.
In code that can assume Python 3.9 or later it is superseded by
<a href="#python-zoneinfo"><code>zoneinfo.ZoneInfo</code></a>
and the <a href="#pypi-tzdata"><code>tzdata</code> Python package</a>.</li>
<li><a href="https://tzinfo.github.io">TZInfo –
Ruby Timezone Library</a>
compiles <code><abbr>tz</abbr></code> source into
<a href="https://www.ruby-lang.org/en/">Ruby</a>.
It is freely available under the <abbr>MIT</abbr> license.</li>
<li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
Library</a> is
a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
library that compiles <code><abbr>tz</abbr></code> source into a time
zone repository whose format
is either proprietary or an <abbr>XML</abbr>-encoded
representation.</li>
<li><a id="Tcl" href="https://tcl.tk">Tcl</a>
contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
source into text files, along with a runtime that can read those
files. Tcl is freely available under a <abbr>BSD</abbr>-style
license.</li>
</ul>
</section>
<section>
<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
<ul>
<li>The <a
href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
Library</a>
has an independent, thread-safe implementation of
a <abbr>TZif</abbr> file reader.
This library is freely available under the LGPL
and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
<li><a href="https://www.gnome.org">GNOME</a>'s
<a href="https://developer.gnome.org/glib/">GLib</a> has
a <abbr>TZif</abbr> file reader written in C that
creates a <code>GTimeZone</code> object representing sets
of <abbr>UT</abbr> offsets.
It is freely available under the <abbr>LGPL</abbr>.</li>
<li>The
<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
<code>baltzo::TimeZoneUtil</code> component contains a C++
implementation of a <abbr>TZif</abbr> file reader. It is freely available under
the Apache License.</li>
<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
library that translates between <abbr>UT</abbr> and civil time and
can read <abbr>TZif</abbr> files. It is freely available under the Apache
License.</li>
<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
library that reads <abbr>TZif</abbr> files and converts
timestamps from one time zone or format to another.
It is used by <a href="https://secure.php.net"><abbr
title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
and <a href="https://www.mongodb.com">MongoDB</a>.
It is freely available under the <abbr>MIT</abbr> license.</li>
<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
<abbr>TZif</abbr> file reader.</li>
<li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
DateTime::TimeZone::Tzfile</a>
is a <abbr>TZif</abbr> file reader written in Perl.
It is freely available under the same terms as Perl
(dual <abbr>GPL</abbr> and Artistic license).</li>
<li>Python has a <a id="python-zoneinfo"
href="https://docs.python.org/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
class</a> that reads <abbr>TZif</abbr> data and creates objects
that represent <code><abbr>tzdb</abbr></code> timezones.
Python is freely available under the
<a href="https://docs.python.org/3.9/license.html">Python Software Foundation
License</a>.
A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
<a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
supplies TZif data if the underlying system data cannot be found;
it is freely available under the Apache License.</li>
<li>The
public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
library contains a Python tool that
converts <abbr>TZif</abbr> data into
<abbr>JSON</abbr>-format data suitable for use
in its JavaScript library for time zone conversion. Dates before 1970
are not supported.</li>
<li>The <a
href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
package contains <a href="https://www.haskell.org">Haskell</a> code that
parses and uses <abbr>TZif</abbr> data. It is freely
available under a <abbr>BSD</abbr>-style license.</li>
</ul>
</section>
<section>
<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
<ul>
<li><a href="https://foxclocks.org">FoxClocks</a>
is an extension for <a href="https://www.google.com/chrome/">Google
Chrome</a> and for <a
href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
Toolkit</a> applications like <a
href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
It displays multiple clocks in the application window, and has a mapping
interface to <a href="https://www.google.com/earth/">Google Earth</a>.
It is freely available under the <abbr>GPL</abbr>.</li>
<li><a href="https://golang.org">Go programming language</a>
implementations contain a copy of a 32-bit subset of a recent
<code><abbr>tz</abbr></code> database in a
Go-specific format.</li>
<li>Microsoft Windows 8.1
and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
data (mentioned <a href="#CLDR">below</a>) used by the
<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
<a id="System.TimeZoneInfo"
href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
the older, proprietary method of Microsoft Windows 2000 and later,
which stores time zone data in the
<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
<a
href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
Tzid table</a> or <a
href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
to <code><abbr>tz</abbr></code> names.
These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
<li><a
href="https://www.oracle.com/java/index.html">Oracle
Java</a> contains a copy of a subset of a recent
<code><abbr>tz</abbr></code> database in a
Java-specific format.</li>
</ul>
</section>
<section>
<h2 id="other-dbs">Other time zone databases</h2>
<ul>
<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
is Astrodienst's Web version of Shanks and Pottenger's out-of-print
time zone history atlases
<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
<a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
Although these extensive atlases
<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
sources for much of the older <code><abbr>tz</abbr></code> data</a>,
they are unreliable as Shanks appears to have
guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
sources and do not indicate which entries are guesswork.</li>
<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
its own <code>tztab</code>(4) format.</li>
<li>Microsoft Windows has proprietary data mentioned
<a href="#System.TimeZoneInfo">above</a>.</li>
<li><a href="https://www.worldtimeserver.com">World Time Server</a>
is another time zone database.</li>
<li>The <a
href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
Schedules Information Manual</a> of the
International Air Transport Association
gives current time zone rules for airports served by commercial aviation.</li>
</ul>
</section>
<section>
<h2 id="maps">Maps</h2>
<ul>
<li>The <a
href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
and Regional Maps section</a> of <em>The World Factbook</em>, published by the
<a href="https://www.cia.gov">US Central Intelligence
Agency (<abbr
title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
zone map; the
<a
href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
Library Map Collection</a>
of the University of Texas at Austin has copies of
recent editions.
The pictorial quality is good,
but the maps do not indicate daylight saving time,
and parts of the data are a few years out of date.</li>
<li><a href="https://www.worldtimezone.com">World Time Zone Map
with current time</a>
has several fancy time zone maps; it covers Russia particularly well.
The maps' pictorial quality is not quite as good as the
<abbr>CIA</abbr>'s
but the maps are more up to date.</li>
<li><a
href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
much is time wrong around the world?</a> maps the difference between
mean solar and standard time, highlighting areas such as western China
where the two differ greatly. It's a bit out of date, unfortunately.</li>
</ul>
</section>
<section>
<h2 id="boundaries">Time zone boundaries</h2>
<p>Geographical boundaries between timezones are available
from several <a
href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
geolocation</a>
services and other sources.</p>
<ul>
<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
Boundary Builder</a> extracts
<a href="https://www.openstreetmap.org">Open Street Map</a> data to build
boundaries of <code><abbr>tzdb</abbr></code> timezones.
Its code is freely available under the <abbr>MIT</abbr> license, and
its data entries are freely available under the
<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
Open Database License</a>. The maps' borders appear to be quite accurate.</li>
<li>Programmatic interfaces that map geographical coordinates via tz_world to
<code><abbr>tzdb</abbr></code> timezones include:
<ul>
<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
written in <a
href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
and is freely available under the <abbr>MIT</abbr> license.</li>
<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
is written in Go and is freely available under the Apache License.</li>
<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
in both Java and
<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
form, is freely available under the MIT license.</li>
<li>For Node.js,
the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
is freely available under the MIT license, and
the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
is in the public domain.</li>
<li>The <a
href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
library for Python is freely available under the MIT license.
<li>The <a
href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
library for Ruby is freely available under the MIT license.</li>
</ul></li>
<li>Free access via a network API, if you register a key, is provided by
the <a
href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
Timezone web service</a>, the <a
href="https://developers.google.com/maps/documentation/timezone/intro">Google
Maps Time Zone API</a>, and
the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
Commercial network API access is provided
by <a href="https://askgeo.com">AskGeo</a>
and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
</li>
<li>"<a
href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
to get a time zone from a location using latitude and longitude
coordinates?</a>" discusses other geolocation possibilities.</li>
<li><a href="http://statoids.com/statoids.html">Administrative
Divisions of Countries ("Statoids")</a> lists
political subdivision data related to time zones.</li>
<li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
zone boundaries for multizone countries</a> summarizes legal
boundaries between time zones within countries.</li>
<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
– GIS and Database Tools</a> includes a Manifold-format map of
world time zone boundaries distributed under the
<abbr>GPL</abbr>.</li>
<li>A ship within the <a
href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
waters</a> of any nation uses that nation's time. In international
waters, time zone boundaries are meridians 15° apart, except that
<abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
wide and are separated by
the 180° meridian (not by the International Date Line, which is
for land and territorial waters only). A captain can change ship's
clocks any time after entering a new time zone; midnight changes are
common.</li>
</ul>
</section>
<section>
<h2 id="civil">Civil time concepts and history</h2>
<ul>
<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
Walk through Time</a>
surveys the evolution of timekeeping.</li>
<li>The history of daylight saving time is surveyed in <a
href="http://www.webexhibits.org/daylightsaving/">About Daylight
Saving Time – History, rationale, laws & dates</a> and summarized in
<a href="http://seizethedaylight.com/dst/">A Brief
History of Daylight Saving Time</a>.</li>
<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
Lords</a> discusses how authoritarians manipulate civil time.</li>
<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
contains guidelines and best practices for software applications that
deal with civil time.</li>
<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
the International Date Line</a> tells the story of the most important
time zone boundary.</li>
<li><a href="http://statoids.com/tconcept.html">Basic Time
Zone Concepts</a> discusses terminological issues behind time zones.</li>
</ul>
</section>
<section>
<h2 id="national">National histories of legal time</h2>
<dl>
<dt>Australia</dt>
<dd>The Parliamentary Library commissioned a <a
href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
paper on daylight saving time in Australia</a>.
The Bureau of Meteorology publishes a list of <a
href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
Dates of Daylight Savings Time within Australia</a>.</dd>
<dt>Belgium</dt>
<dd>The Royal Observatory of Belgium maintains a table of time in
Belgium (in
<a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
hreflang="nl">Dutch</a> and <a
href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
hreflang="fr">French</a>).</dd>
<dt>Brazil</dt>
<dd>The Time Service Department of the National Observatory
records <a href="http://pcdsh01.on.br/DecHV.html"
hreflang="pt-BR">Brazil's daylight saving time decrees (in
Portuguese)</a>.</dd>
<dt>Canada</dt>
<dd>National Research Council Canada publishes current
and some older information about <a
href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
zones and daylight saving time</a>.</dd>
<dt>Chile</dt>
<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
<a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
Chile's official time (in Spanish)</a>.</dd>
<dt>China</dt>
<dd>The Hong Kong Observatory maintains a
<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
summer time in Hong Kong</a>,
and Macau's Meteorological and Geophysical Bureau maintains a <a
href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
history for Macau</a>.
Unfortunately the latter is incomplete and has errors.</dd>
<dt>Czech Republic</dt>
<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
<dt>Germany</dt>
<dd>The National Institute for Science and Technology maintains the <a
href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
of Legal Time in Germany</a>.</dd>
<dt>Israel</dt>
<dd>The Interior Ministry periodically issues <a
href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
hreflang="he">announcements (in Hebrew)</a>.</dd>
<dt>Malaysia</dt>
<dd>See Singapore <a href="#Singapore">below</a>.</dd>
<dt>Mexico</dt>
<dd>The Investigation and Analysis Service of the Mexican Library of
Congress has published a <a
href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
<dt>Netherlands</dt>
<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
covers the history of local time in the Netherlands from ancient times.</dd>
<dt>New Zealand</dt>
<dd>The Department of Internal Affairs maintains a brief <a
href="https://www.dia.govt.nz/Daylight-Saving-History">History of
Daylight Saving</a>.</dd>
<dt>Portugal</dt>
<dd>The Lisbon Astronomical Observatory publishes a
<a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
legal time (in Portuguese)</a>.</dd>
<dt>Singapore</dt>
<dd><a id="Singapore"
href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
is Singapore in the "Wrong" Time Zone?</a> details the
history of legal time in Singapore and Malaysia.</dd>
<dt>United Kingdom</dt>
<dd><a
href="https://www.polyomino.org.uk/british-time/">History of
legal time in Britain</a> discusses in detail the country
with perhaps the best-documented history of clock adjustments.</dd>
<dt>United States</dt>
<dd>The Department of Transportation's <a
href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
<dt>Uruguay</dt>
<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
Navy (SOHMA) publishes an annual <a
href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
(in Spanish)</a>.</dd>
</dl>
</section>
<section>
<h2 id="costs">Costs and benefits of time shifts</h2>
<p>Various sources argue for and against daylight saving time and time
zone shifts, and many scientific studies have been conducted. This
section summarizes reviews and position statements based on
scientific literature in the area.</p>
<ul>
<li>Carey RN, Sarma KM.
<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
daylight saving time on road traffic collision risk: a systematic
review</a>.
<em>BMJ Open.</em> 2017;7(6):e014319. doi:<a href="https://doi.org/10.1136/bmjopen-2016-014319">10.1136/bmjopen-2016-014319</a>.
This reviews research literature and concludes that the evidence
neither supports nor refutes road safety benefits from
shifts in time zones.</li>
<li>Havranek T, Herman D, Irsova D.
<a href="https://www.econstor.eu/handle/10419/174191">Does daylight
saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
2018;39(2).
doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
This analyzes research literature and concludes, "Electricity savings
are larger for countries farther away from the equator, while
subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
<li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
an American Academy of Sleep Medicine position statement</a>.
<em>J Clin Sleep Med.</em>
2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
This argues for permanent standard time due to health risks of both
<abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
<li>Roenneberg T, Winnebeck EC, Klerman EB.
<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
saving time and artificial time zones – a battle between
biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
This reviews evidence about the health effects of <abbr>DST</abbr>
and concludes,
"In summary, the scientific literature strongly argues against the
switching between <abbr>DST</abbr> and Standard Time and even more so against
adopting <abbr>DST</abbr> permanently."</li>
</ul>
</section>
<section>
<h2 id="precision">Precision timekeeping</h2>
<ul>
<li><a
href="http://leapsecond.com/hpan/an1289.pdf">The
Science of Timekeeping</a> is a thorough introduction
to the theory and practice of precision timekeeping.</li>
<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
Time 2016</a> contains several freely-readable papers.</li>
<li><a href="http://www.ntp.org"><abbr
title="Network Time Protocol">NTP</abbr>: The Network
Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
discusses how to synchronize clocks of
Internet hosts.</li>
<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
family of software algorithms can achieve accuracy to a few tens of
nanoseconds in scalable server farms without special hardware.</li>
<li>The <a
href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision