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1 files changed, 27 insertions, 10 deletions
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index 677baf6f8c720..ea4b35d8137f0 100644
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@@ -8,6 +8,7 @@ Theory and pragmatics of the tz code and data
Time zone abbreviations
Accuracy of the tz database
Time and date functions
+ Interface stability
Calendrical issues
Time and time zones on Mars
@@ -342,12 +343,24 @@ Errors in the tz database arise from many sources:
non-hour-based system at night.
* Early clocks were less reliable, and data entries do not represent
- this unreliability.
-
- * As for leap seconds, civil time was not based on atomic time before
- 1972, and we don't know the history of earth's rotation accurately
- enough to map SI seconds to historical solar time to more than
- about one-hour accuracy. See: Morrison LV, Stephenson FR.
+ clock error.
+
+ * The tz database assumes Universal Time (UT) as an origin, even
+ though UT is not standardized for older time stamps. In the tz
+ database commentary, UT denotes a family of time standards that
+ includes Coordinated Universal Time (UTC) along with other variants
+ such as UT1 and GMT, with days starting at midnight. Although UT
+ equals UTC for modern time stamps, UTC was not defined until 1960,
+ so commentary uses the more-general abbreviation UT for time stamps
+ that might predate 1960. Since UT, UT1, etc. disagree slightly,
+ and since pre-1972 UTC seconds varied in length, interpretation of
+ older time stamps can be problematic when subsecond accuracy is
+ needed.
+
+ * Civil time was not based on atomic time before 1972, and we don't
+ know the history of earth's rotation accurately enough to map SI
+ seconds to historical solar time to more than about one-hour
+ accuracy. See: Morrison LV, Stephenson FR.
Historical values of the Earth's clock error Delta T and the
calculation of eclipses. J Hist Astron. 2004;35:327-36
<http://adsabs.harvard.edu/full/2004JHA....35..327M>;
@@ -601,10 +614,14 @@ The tz code and data supply the following interfaces:
* The format of the country code file, documented in iso3166.tab.
-When these interfaces are changed, an effort is made to preserve
-backward compatibility. For example, tz data files typically do not
-rely on recently-added zic features, so that users can run older zic
-versions to process newer data files.
+ * The version number of the code and data, as the first line of
+ the text file 'version' in each release.
+
+Interface changes in a release attempt to preserve compatibility with
+recent releases. For example, tz data files typically do not rely on
+recently-added zic features, so that users can run older zic versions
+to process newer data files. The tz-link.htm file describes how
+releases are tagged and distributed.
Interfaces not listed above are less stable. For example, users
should not rely on particular UT offsets or abbreviations for time