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Diffstat (limited to 'theory.html')
| -rw-r--r-- | theory.html | 27 |
1 files changed, 21 insertions, 6 deletions
diff --git a/theory.html b/theory.html index 5a05f4b12d22..28cc88eafdef 100644 --- a/theory.html +++ b/theory.html @@ -296,7 +296,7 @@ in decreasing order of importance: </li> <li> If a name is changed, put its old spelling in the - '<code>backward</code>' file. + '<code>backward</code>' file as a link to the new spelling. This means old spellings will continue to work. Ordinarily a name change should occur only in the rare case when a location's consensus English-language spelling changes; for example, @@ -348,22 +348,37 @@ timestamps correctly and it increased maintenance burden. </ul> <p> -The file '<code>zone1970.tab</code>' lists geographical locations used +The file <code>zone1970.tab</code> lists geographical locations used to name timezones. It is intended to be an exhaustive list of names for geographic regions as described above; this is a subset of the timezones in the data. -Although a '<code>zone1970.tab</code>' location's +Although a <code>zone1970.tab</code> location's <a href="https://en.wikipedia.org/wiki/Longitude">longitude</a> corresponds to its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean time (<abbr>LMT</abbr>)</a> offset with one hour for every 15° east longitude, this relationship is not exact. +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>. </p> <p> -Excluding '<code>backward</code>' should not affect the other data. -If '<code>backward</code>' is excluded, excluding -'<code>etcetera</code>' should not affect the remaining data. +The database defines each timezone name to be a zone, or a link to a zone. +The source file <code>backward</code> defines links for backward +compatibility; it does not define zones. +Although <code>backward</code> was originally designed to be optional, +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; +no other source file other than <code>backward</code> +contains links to its zones. +One of <code>etcetera</code>'s names is <code>GMT</code>, +used by functions like <code>gmtime</code> to obtain leap +second information on platforms that support leap seconds. </p> </section> |