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Diffstat (limited to 'tz/theory.html')
| -rw-r--r-- | tz/theory.html | 39 |
1 files changed, 20 insertions, 19 deletions
diff --git a/tz/theory.html b/tz/theory.html index 28f6b8e..5a05f4b 100644 --- a/tz/theory.html +++ b/tz/theory.html @@ -42,12 +42,13 @@ href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a> (1970-01-01 00:00:00 <a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr title="Coordinated Universal Time">UTC</abbr></a>). -The database labels each timezone with a notable location and -records all known clock transitions for that location. Although 1970 is a somewhat-arbitrary cutoff, there are significant challenges to moving the cutoff earlier even by a decade or two, due to the wide variety of local practices before computer timekeeping became prevalent. +Most timezones correspond to a notable location and the database +records all known clock transitions for that location; +some timezones correspond instead to a fixed <abbr>UTC</abbr> offset. </p> <p> @@ -58,8 +59,9 @@ specifies current standard time. For example, applications that deal with current and future timestamps in the traditional North American mountain time zone can choose from the timezones <code>America/Denver</code> which observes US-style daylight saving -time, <code>America/Mazatlan</code> which observes Mexican-style DST, -and <code>America/Phoenix</code> which does not observe DST. +time (<abbr>DST</abbr>), +<code>America/Mazatlan</code> which observes Mexican-style <abbr>DST</abbr>, +and <code>America/Phoenix</code> which does not observe <abbr>DST</abbr>. Applications that also deal with past timestamps in the mountain time zone can choose from over a dozen timezones, such as <code>America/Boise</code>, <code>America/Edmonton</code>, and @@ -68,7 +70,7 @@ time but differs from other timezones for some timestamps after 1970. </p> <p> -Clock transitions before 1970 are recorded for each timezone, +Clock transitions before 1970 are recorded for location-based timezones, because most systems support timestamps before 1970 and could misbehave if data entries were omitted for pre-1970 transitions. However, the database is not designed for and does not suffice for @@ -190,8 +192,8 @@ in decreasing order of importance: <code>TZ</code> strings</a>. A file name component must not exceed 14 characters or start with '<code>-</code>'. - E.g., prefer <code>Asia/Brunei</code> to - <code>Asia/Bandar_Seri_Begawan</code>. + E.g., prefer <code>America/Noronha</code> to + <code>America/Fernando_de_Noronha</code>. Exceptions: see the discussion of legacy names below. </li> <li> @@ -473,10 +475,10 @@ in decreasing order of importance: <p> <small>These abbreviations are: - AMT Amsterdam, Asunción, Athens; + AMT Asunción, Athens; BMT Baghdad, Bangkok, Batavia, Bermuda, Bern, Bogotá, Bridgetown, Brussels, Bucharest; - CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba; + CMT Calamarca, Caracas, Chisinau, Colón, Córdoba; DMT Dublin/Dunsink; EMT Easter; FFMT Fort-de-France; @@ -499,7 +501,8 @@ in decreasing order of importance: SMT Santiago, Simferopol, Singapore, Stanley; TBMT Tbilisi; TMT Tallinn, Tehran; - WMT Warsaw</small>. + WMT Warsaw; + ZMT Zomba.</small> </p> <p> @@ -513,9 +516,7 @@ in decreasing order of importance: 1880–1916, MMT/MST/MDST for Moscow 1880–1919, and RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926. - An extra-special case is SET for Swedish Time (<em>svensk - normaltid</em>) 1879–1899, 3° west of the Stockholm - Observatory.</small> + </small> </p> </li> <li> @@ -702,11 +703,9 @@ href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanes <li> Sometimes historical timekeeping was specified more precisely than what the <code><abbr>tz</abbr></code> code can handle. - For example, from 1909 to 1937 <a - href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" - hreflang="nl">Netherlands clocks</a> were legally Amsterdam Mean + For example, from 1880 to 1916 clocks in Ireland observed Dublin Mean Time (estimated to be <abbr>UT</abbr> - +00:19:32.13), but the <code><abbr>tz</abbr></code> + −00:25:21.1), but the <code><abbr>tz</abbr></code> code cannot represent the fractional second. In practice these old specifications were rarely if ever implemented to subsecond precision. @@ -753,7 +752,8 @@ href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanes the Western 06:00 to be 12:00. These practices are largely outside the scope of the <code><abbr>tz</abbr></code> code and data, which provide only limited support for date and time localization - such as that required by POSIX. If DST is not used a different time zone + such as that required by POSIX. + If <abbr>DST</abbr> is not used a different time zone can often do the trick; for example, in Kenya a <code>TZ</code> setting like <code><-03>3</code> or <code>America/Cayenne</code> starts the day six hours later than <code>Africa/Nairobi</code> does. @@ -1271,7 +1271,8 @@ The directly-supported mechanism assumes that <code>time_t</code> counts of seconds since the POSIX epoch normally include leap seconds, as opposed to POSIX <code>time_t</code> counts which exclude leap seconds. This modified timescale is converted to <abbr>UTC</abbr> -at the same point that time zone and DST adjustments are applied – +at the same point that time zone and <abbr>DST</abbr> +adjustments are applied – namely, at calls to <code>localtime</code> and analogous functions – and the process is driven by leap second information stored in alternate versions of the <abbr>TZif</abbr> files. |