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Diff for /src/lib/libc/time/Theory between version 1.22 and 1.23

version 1.22, 2016/10/20 17:41:34 version 1.23, 2017/03/11 18:23:14
Line 8  Theory and pragmatics of the tz code and
Line 8  Theory and pragmatics of the tz code and
         Time zone abbreviations          Time zone abbreviations
         Accuracy of the tz database          Accuracy of the tz database
         Time and date functions          Time and date functions
           Interface stability
         Calendrical issues          Calendrical issues
         Time and time zones on Mars          Time and time zones on Mars
   
Line 39  international standard for UNIX-like sys
Line 40  international standard for UNIX-like sys
 current edition of POSIX is:  current edition of POSIX is:
   
   The Open Group Base Specifications Issue 7    The Open Group Base Specifications Issue 7
   IEEE Std 1003.1, 2013 Edition    IEEE Std 1003.1-2008, 2016 Edition
   <http://pubs.opengroup.org/onlinepubs/9699919799/>    <http://pubs.opengroup.org/onlinepubs/9699919799/>
   
   
Line 208  in decreasing order of importance:
Line 209  in decreasing order of importance:
         If there is no common English abbreviation, use numeric offsets like          If there is no common English abbreviation, use numeric offsets like
                 -05 and +0830 that are generated by zic's %z notation.                  -05 and +0830 that are generated by zic's %z notation.
   
           Use current abbreviations for older timestamps to avoid confusion.
                   For example, in 1910 a common English abbreviation for UT +01
                   in central Europe was 'MEZ' (short for both "Middle European
                   Zone" and for "Mitteleuropäische Zeit" in German).  Nowadays
                   'CET' ("Central European Time") is more common in English, and
                   the database uses 'CET' even for circa-1910 timestamps as this
                   is less confusing for modern users and avoids the need for
                   determining when 'CET' supplanted 'MEZ' in common usage.
   
           Use a consistent style in a zone's history.  For example, if a zone's
                   history tends to use numeric abbreviations and a particular
                   entry could go either way, use a numeric abbreviation.
   
     [The remaining guidelines predate the introduction of %z.      [The remaining guidelines predate the introduction of %z.
     They are problematic as they mean tz data entries invent      They are problematic as they mean tz data entries invent
     notation rather than record it.  These guidelines are now      notation rather than record it.  These guidelines are now
Line 225  in decreasing order of importance:
Line 239  in decreasing order of importance:
                         for double summer time append 'DST'; etc.                          for double summer time append 'DST'; etc.
                 Otherwise, take the first three letters of an English place                  Otherwise, take the first three letters of an English place
                         name identifying each zone and append 'T', 'ST', etc.                          name identifying each zone and append 'T', 'ST', etc.
                         as before; e.g. 'VLAST' for VLAdivostok Summer Time.                          as before; e.g. 'CHAST' for CHAtham Summer Time.
   
         Use UT (with time zone abbreviation '-00') for locations while          Use UT (with time zone abbreviation '-00') for locations while
                 uninhabited.  The leading '-' is a flag that the time                  uninhabited.  The leading '-' is a flag that the time
Line 342  Errors in the tz database arise from man
Line 356  Errors in the tz database arise from man
    non-hour-based system at night.     non-hour-based system at night.
   
  * Early clocks were less reliable, and data entries do not represent   * Early clocks were less reliable, and data entries do not represent
    this unreliability.     clock error.
   
  * As for leap seconds, civil time was not based on atomic time before   * The tz database assumes Universal Time (UT) as an origin, even
    1972, and we don't know the history of earth's rotation accurately     though UT is not standardized for older time stamps.  In the tz
    enough to map SI seconds to historical solar time to more than     database commentary, UT denotes a family of time standards that
    about one-hour accuracy.  See: Morrison LV, Stephenson FR.     includes Coordinated Universal Time (UTC) along with other variants
    Historical values of the Earth's clock error Delta T and the     such as UT1 and GMT, with days starting at midnight.  Although UT
    calculation of eclipses. J Hist Astron. 2004;35:327-36     equals UTC for modern time stamps, UTC was not defined until 1960,
    <http://adsabs.harvard.edu/full/2004JHA....35..327M>;     so commentary uses the more-general abbreviation UT for time stamps
    Historical values of the Earth's clock error. J Hist Astron. 2005;36:339     that might predate 1960.  Since UT, UT1, etc. disagree slightly,
    <http://adsabs.harvard.edu/full/2005JHA....36..339M>.     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: Stephenson FR, Morrison LV, Hohenkerk CY.
      Measurement of the Earth's rotation: 720 BC to AD 2015.
      Proc Royal Soc A. 2016 Dec 7;472:20160404.
      http://dx.doi.org/10.1098/rspa.2016.0404
      Also see: Espenak F. Uncertainty in Delta T (T).
      http://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html
   
  * The relationship between POSIX time (that is, UTC but ignoring leap   * The relationship between POSIX time (that is, UTC but ignoring leap
    seconds) and UTC is not agreed upon after 1972.  Although the POSIX     seconds) and UTC is not agreed upon after 1972.  Although the POSIX
Line 601  The tz code and data supply the followin
Line 627  The tz code and data supply the followin
   
  * The format of the country code file, documented in iso3166.tab.   * The format of the country code file, documented in iso3166.tab.
   
 When these interfaces are changed, an effort is made to preserve   * The version number of the code and data, as the first line of
 backward compatibility.  For example, tz data files typically do not     the text file 'version' in each release.
 rely on recently-added zic features, so that users can run older zic  
 versions to process newer data files.  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  Interfaces not listed above are less stable.  For example, users
 should not rely on particular UT offsets or abbreviations for time  should not rely on particular UT offsets or abbreviations for time
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