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Diffstat (limited to 'contrib/tzdata/Theory')
| -rw-r--r-- | contrib/tzdata/Theory | 29 |
1 files changed, 21 insertions, 8 deletions
diff --git a/contrib/tzdata/Theory b/contrib/tzdata/Theory index ea4b35d..33e4606 100644 --- a/contrib/tzdata/Theory +++ b/contrib/tzdata/Theory @@ -40,7 +40,7 @@ international standard for UNIX-like systems. As of this writing, the current edition of POSIX is: 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/> @@ -209,6 +209,19 @@ in decreasing order of importance: If there is no common English abbreviation, use numeric offsets like -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. They are problematic as they mean tz data entries invent notation rather than record it. These guidelines are now @@ -226,7 +239,7 @@ in decreasing order of importance: for double summer time append 'DST'; etc. Otherwise, take the first three letters of an English place 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 uninhabited. The leading '-' is a flag that the time @@ -360,12 +373,12 @@ Errors in the tz database arise from many sources: * 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>; - Historical values of the Earth's clock error. J Hist Astron. 2005;36:339 - <http://adsabs.harvard.edu/full/2005JHA....36..339M>. + 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 (ΔT). + http://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html * The relationship between POSIX time (that is, UTC but ignoring leap seconds) and UTC is not agreed upon after 1972. Although the POSIX |
