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Diffstat (limited to 'theory.html')
| -rw-r--r-- | theory.html | 37 |
1 files changed, 21 insertions, 16 deletions
diff --git a/theory.html b/theory.html index 369c75433ff2..516d2a525111 100644 --- a/theory.html +++ b/theory.html @@ -95,7 +95,7 @@ Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018 Edition. Because the database's scope encompasses real-world changes to civil timekeeping, its model for describing time is more complex than the -standard and daylight saving times supported by POSIX. +standard and daylight saving times supported by POSIX.1-2017. A <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can have more than two changes per year, these changes need not merely flip back and forth between two alternatives, and the rules themselves @@ -187,7 +187,7 @@ in decreasing order of importance: href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters, '<code>.</code>', '<code>-</code>' and '<code>_</code>'. Do not use digits, as that might create an ambiguity with <a - href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX + href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX.1-2017 <code>TZ</code> strings</a>. A file name component must not exceed 14 characters or start with '<code>-</code>'. @@ -362,6 +362,11 @@ The backward-compatibility file <code>zone.tab</code> is similar but conforms to the older-version guidelines related to <abbr>ISO</abbr> 3166-1; it lists only one country code per entry and unlike <code>zone1970.tab</code> it can list names defined in <code>backward</code>. +Applications that process only timestamps from now on can instead use the file +<code>zonenow.tab</code>, which partitions the world more coarsely, +into regions where clocks agree now and in the predicted future; +this file is smaller and simpler than <code>zone1970.tab</code> +and <code>zone.tab</code>. </p> <p> @@ -373,7 +378,7 @@ nowadays distributions typically use it and no great weight should be attached to whether a link is defined in <code>backward</code> or in some other file. The source file <code>etcetera</code> defines names that may be useful -on platforms that do not support POSIX-style <code>TZ</code> strings; +on platforms that do not support POSIX.1-2017-style <code>TZ</code> strings; no other source file other than <code>backward</code> contains links to its zones. One of <code>etcetera</code>'s names is <code>Etc/UTC</code>, @@ -421,7 +426,7 @@ in decreasing order of importance: expression <code>[-+[:alnum:]]{3,6}</code> should match the abbreviation. This guarantees that all abbreviations could have been specified by a - POSIX <code>TZ</code> string. + POSIX.1-2017 <code>TZ</code> string. </p> </li> <li> @@ -765,12 +770,12 @@ href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanes calendar with 24-hour days. These divergences range from relatively minor, such as Japanese bars giving times like "24:30" for the wee hours of the morning, to more-significant differences such as <a - href="https://www.pri.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the + href="https://theworld.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the east African practice of starting the day at dawn</a>, renumbering 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. + such as that required by POSIX.1-2017. 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 @@ -867,23 +872,23 @@ input is occasionally extended, and a platform may still be shipping an older <code>zic</code>. </p> -<h3 id="POSIX">POSIX properties and limitations</h3> +<h3 id="POSIX">POSIX.1-2017 properties and limitations</h3> <ul> <li> <p> - In POSIX, time display in a process is controlled by the + In POSIX.1-2017, time display in a process is controlled by the environment variable <code>TZ</code>. - Unfortunately, the POSIX + Unfortunately, the POSIX.1-2017 <code>TZ</code> string takes a form that is hard to describe and is error-prone in practice. - Also, POSIX <code>TZ</code> strings cannot deal with daylight + Also, POSIX.1-2017 <code>TZ</code> strings cannot deal with daylight saving time rules not based on the Gregorian calendar (as in Morocco), or with situations where more than two time zone abbreviations or <abbr>UT</abbr> offsets are used in an area. </p> <p> - The POSIX <code>TZ</code> string takes the following form: + The POSIX.1-2017 <code>TZ</code> string takes the following form: </p> <p> @@ -950,7 +955,7 @@ an older <code>zic</code>. </dl> <p> - Here is an example POSIX <code>TZ</code> string for New + Here is an example POSIX.1-2017 <code>TZ</code> string for New Zealand after 2007. It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead of <abbr>UT</abbr>, and that daylight saving time @@ -961,7 +966,7 @@ an older <code>zic</code>. <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> <p> - This POSIX <code>TZ</code> string is hard to remember, and + This POSIX.1-2017 <code>TZ</code> string is hard to remember, and mishandles some timestamps before 2008. With this package you can use this instead: </p> @@ -999,7 +1004,7 @@ an older <code>zic</code>. limit phone calls to off-peak hours. </li> <li> - POSIX provides no convenient and efficient way to determine + POSIX.1-2017 provides no convenient and efficient way to determine the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary timestamps, particularly for timezones that do not fit into the POSIX model. @@ -1026,14 +1031,14 @@ an older <code>zic</code>. </li> </ul> -<h3 id="POSIX-extensions">Extensions to POSIX in the +<h3 id="POSIX-extensions">Extensions to POSIX.1-2017 in the <code><abbr>tz</abbr></code> code</h3> <ul> <li> <p> The <code>TZ</code> environment variable is used in generating the name of a file from which time-related information is read - (or is interpreted à la POSIX); <code>TZ</code> is no longer + (or is interpreted à la POSIX.1-2017); <code>TZ</code> is no longer constrained to be a string containing abbreviations and numeric data as described <a href="#POSIX">above</a>. The file's format is <dfn><abbr>TZif</abbr></dfn>, |