@@ -8,6 +8,7 @@ Theory and pragmatics of the tz code and data
88 Time zone abbreviations
99 Accuracy of the tz database
1010 Time and date functions
11+ Interface stability
1112 Calendrical issues
1213 Time and time zones on Mars
1314
@@ -342,12 +343,24 @@ Errors in the tz database arise from many sources:
342343 non-hour-based system at night.
343344
344345 * Early clocks were less reliable, and data entries do not represent
345- this unreliability.
346-
347- * As for leap seconds, civil time was not based on atomic time before
348- 1972, and we don't know the history of earth's rotation accurately
349- enough to map SI seconds to historical solar time to more than
350- about one-hour accuracy. See: Morrison LV, Stephenson FR.
346+ clock error.
347+
348+ * The tz database assumes Universal Time (UT) as an origin, even
349+ though UT is not standardized for older time stamps. In the tz
350+ database commentary, UT denotes a family of time standards that
351+ includes Coordinated Universal Time (UTC) along with other variants
352+ such as UT1 and GMT, with days starting at midnight. Although UT
353+ equals UTC for modern time stamps, UTC was not defined until 1960,
354+ so commentary uses the more-general abbreviation UT for time stamps
355+ that might predate 1960. Since UT, UT1, etc. disagree slightly,
356+ and since pre-1972 UTC seconds varied in length, interpretation of
357+ older time stamps can be problematic when subsecond accuracy is
358+ needed.
359+
360+ * Civil time was not based on atomic time before 1972, and we don't
361+ know the history of earth's rotation accurately enough to map SI
362+ seconds to historical solar time to more than about one-hour
363+ accuracy. See: Morrison LV, Stephenson FR.
351364 Historical values of the Earth's clock error Delta T and the
352365 calculation of eclipses. J Hist Astron. 2004;35:327-36
353366 <http://adsabs.harvard.edu/full/2004JHA....35..327M>;
@@ -601,10 +614,14 @@ The tz code and data supply the following interfaces:
601614
602615 * The format of the country code file, documented in iso3166.tab.
603616
604- When these interfaces are changed, an effort is made to preserve
605- backward compatibility. For example, tz data files typically do not
606- rely on recently-added zic features, so that users can run older zic
607- versions to process newer data files.
617+ * The version number of the code and data, as the first line of
618+ the text file 'version' in each release.
619+
620+ Interface changes in a release attempt to preserve compatibility with
621+ recent releases. For example, tz data files typically do not rely on
622+ recently-added zic features, so that users can run older zic versions
623+ to process newer data files. The tz-link.htm file describes how
624+ releases are tagged and distributed.
608625
609626Interfaces not listed above are less stable. For example, users
610627should not rely on particular UT offsets or abbreviations for time
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