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===================================================================
RCS file: /ftp/cvs/cvsroot/src/lib/libc/time/Theory,v
rcsdiff: /ftp/cvs/cvsroot/src/lib/libc/time/Theory,v: warning: Unknown phrases like `commitid ...;' are present.
retrieving revision 1.22
retrieving revision 1.23
diff -u -p -r1.22 -r1.23
--- src/lib/libc/time/Theory	2016/10/20 17:41:34	1.22
+++ src/lib/libc/time/Theory	2017/03/11 18:23:14	1.23
@@ -8,6 +8,7 @@ Theory and pragmatics of the tz code and
 	Time zone abbreviations
 	Accuracy of the tz database
 	Time and date functions
+	Interface stability
 	Calendrical issues
 	Time and time zones on Mars
 
@@ -39,7 +40,7 @@ international standard for UNIX-like sys
 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/>
 
 
@@ -208,6 +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
@@ -225,7 +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
@@ -342,17 +356,29 @@ Errors in the tz database arise from man
    non-hour-based system at night.
 
  * 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
-   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>.
+ * 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: 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
    seconds) and UTC is not agreed upon after 1972.  Although the POSIX
@@ -601,10 +627,14 @@ The tz code and data supply the followin
 
  * 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