Improve time zone documentation

* doc/lispref/os.texi (Time Zone Rules):
New section, mostly with material moved here from other sections.
* doc/emacs/cmdargs.texi (General Variables):
* doc/lispref/os.texi (Time Conversion, Time Parsing):
Xref new section.
* etc/NEWS, etc/PROBLEMS:
* lisp/org/org.el (org-timestamp-format):
* src/editfns.c (Fformat_time_string, Fdecode_time)
(Fencode_time, Fcurrent_time_string, Fcurrent_time_zone)
(Fset_time_zone_rule):
When documenting time zone rule strings, mention the TZ
environment variable in preference to mentioning the
sort-of-internal function set-time-zone-rule.

doc/emacs/cmdargs.texi

@@ -634,8 +634,9 @@ to put temporary files (@pxref{Backup}).  Emacs tries to use
 @env{TMP}, then @env{TEMP}, and finally @file{c:/temp}.
 @item TZ
 @vindex TZ, environment variable
-This specifies the current time zone and possibly also daylight
+This specifies the default time zone and possibly also daylight
-saving time information.  On MS-DOS, if @env{TZ} is not set in the
+saving time information.  @xref{Time Zone Rules,,, elisp, The GNU
+Emacs Lisp Reference Manual}.  On MS-DOS, if @env{TZ} is not set in the
 environment when Emacs starts, Emacs defines a default value as
 appropriate for the country code returned by DOS@.  On MS-Windows, Emacs
 does not use @env{TZ} at all.

doc/lispref/os.texi

@@ -19,6 +19,7 @@ terminal and the screen.
 * System Environment::  Distinguish the name and kind of system.
 * User Identification:: Finding the name and user id of the user.
 * Time of Day::         Getting the current time.
+* Time Zone Rules::     Rules for time zones and daylight saving time.
 * Time Conversion::     Converting a time from numeric form to
                           calendrical data and vice versa.
 * Time Parsing::        Converting a time from numeric form to text
@@ -1262,7 +1263,7 @@ information may some day be added at the end.
 
 The argument @var{time}, if given, specifies a time to format,
 instead of the current time.  The optional argument @var{zone}
-defaults to the current time zone rule.
+defaults to the current time zone rule.  @xref{Time Zone Rules}.
 
 @example
 @group
@@ -1299,26 +1300,9 @@ For example, if @var{time} is a number, @code{(time-to-seconds
 or rounding errors occur.
 @end defun
 
-@defun current-time-zone &optional time zone
+@node Time Zone Rules
-@cindex time zone, current
+@section Time Zone Rules
-This function returns a list describing the time zone that the user is
+@cindex time zone rules
-in.
-
-The value has the form @code{(@var{offset} @var{name})}.  Here
-@var{offset} is an integer giving the number of seconds ahead of Universal Time
-(east of Greenwich).  A negative value means west of Greenwich.  The
-second element, @var{name}, is a string giving the name of the time
-zone.  Both elements change when daylight saving time begins or ends;
-if the user has specified a time zone that does not use a seasonal time
-adjustment, then the value is constant through time.
-
-If the operating system doesn't supply all the information necessary to
-compute the value, the unknown elements of the list are @code{nil}.
-
-The argument @var{time}, if given, specifies a time value to
-analyze instead of the current time.  The optional argument @var{zone}
-defaults to the current time zone rule.
-@end defun
 
 @vindex TZ, environment variable
 The default time zone is determined by the @env{TZ} environment
@@ -1327,6 +1311,15 @@ to default to Universal Time with @code{(setenv "TZ" "UTC0")}.  If
 @env{TZ} is not in the environment, Emacs uses system wall clock time,
 which is a platform-dependent default time zone.
 
+The set of supported @env{TZ} strings is system-dependent.  GNU and
+many other systems support the tzdata database, e.g.,
+@samp{"America/New_York"} specifies the time zone and daylight saving
+time history for locations near New York City.  GNU and most other
+systems support POSIX-style @env{TZ} strings, e.g.,
+@samp{"EST+5EDT,M4.1.0/2,M10.5.0/2"} specifies the rules used in New
+York from 1987 through 2006.  All systems support the string
+@samp{"UTC0"} meaning Universal Time.
+
 @cindex time zone rule
 Functions that convert to and from local time accept an optional
 @dfn{time zone rule} argument, which specifies the conversion's time
@@ -1337,6 +1330,29 @@ If it is @code{t}, the conversion uses Universal Time.  If it is
 a string, the conversion uses the time zone rule equivalent to setting
 @env{TZ} to that string.
 
+@defun current-time-zone &optional time zone
+@cindex time zone, current
+This function returns a list describing the time zone that the user is
+in.
+
+The value has the form @code{(@var{offset} @var{abbr})}.  Here
+@var{offset} is an integer giving the number of seconds ahead of Universal Time
+(east of Greenwich).  A negative value means west of Greenwich.  The
+second element, @var{abbr}, is a string giving an abbreviation for the
+time zone, e.g., @samp{"CST"} for China Standard Time or for
+U.S. Central Standard Time.  Both elements can change when daylight
+saving time begins or ends; if the user has specified a time zone that
+does not use a seasonal time adjustment, then the value is constant
+through time.
+
+If the operating system doesn't supply all the information necessary to
+compute the value, the unknown elements of the list are @code{nil}.
+
+The argument @var{time}, if given, specifies a time value to
+analyze instead of the current time.  The optional argument @var{zone}
+defaults to the current time zone rule.
+@end defun
+
 @node Time Conversion
 @section Time Conversion
 @cindex calendrical information
@@ -1361,8 +1377,8 @@ as traditional Gregorian years do; for example, the year number
 @defun decode-time &optional time zone
 This function converts a time value into calendrical information.  If
 you don't specify @var{time}, it decodes the current time, and similarly
-@var{zone} defaults to the current time zone rule.  The return
+@var{zone} defaults to the current time zone rule.  @xref{Time Zone Rules}.
-value is a list of nine elements, as follows:
+The return value is a list of nine elements, as follows:
 
 @example
 (@var{seconds} @var{minutes} @var{hour} @var{day} @var{month} @var{year} @var{dow} @var{dst} @var{utcoff})
@@ -1409,6 +1425,7 @@ to stand for years above 1900, or years above 2000, you must alter them
 yourself before you call @code{encode-time}.
 
 The optional argument @var{zone} defaults to the current time zone rule.
+@xref{Time Zone Rules}.
 In addition to the usual time zone rule values, it can also be a list
 (as you would get from @code{current-time-zone}) or an integer (as
 from @code{decode-time}), applied without any further alteration for
@@ -1452,8 +1469,8 @@ corresponding time value.
 
 This function converts @var{time} (or the current time, if
 @var{time} is omitted) to a string according to
-@var{format-string}.  The conversion uses the time zone rule @var{zone}
+@var{format-string}.  The conversion uses the time zone rule @var{zone}, which
-(or the current time zone rule, if omitted).  The argument
+defaults to the current time zone rule.  @xref{Time Zone Rules}.  The argument
 @var{format-string} may contain @samp{%}-sequences which say to
 substitute parts of the time.  Here is a table of what the
 @samp{%}-sequences mean:

etc/NEWS

@@ -1778,11 +1778,11 @@ quotation marks.
 *** Time conversion functions now accept an optional ZONE argument
 that specifies the time zone rules for conversion.  ZONE is omitted or
 nil for Emacs local time, t for Universal Time, 'wall' for system wall
-clock time, or a string as in 'set-time-zone-rule' for a time zone
+clock time, or a string as in the TZ environment variable.  The
-rule.  The affected functions are 'current-time-string',
+affected functions are 'current-time-string', 'current-time-zone',
-'current-time-zone', 'decode-time', and 'format-time-string'.  The
+'decode-time', and 'format-time-string'.  The function 'encode-time',
-function 'encode-time', which already accepted a simple time zone rule
+which already accepted a simple time zone rule argument, has been
-argument, has been extended to accept all the new forms.
+extended to accept all the new forms.
 
 *** Incompatible change in the third argument of 'format-time-string'.
 Previously, any non-nil argument was interpreted as specifying Universal Time.

etc/PROBLEMS

@@ -2220,11 +2220,11 @@ month names with consistent widths for some locales on some versions
 of Windows.  This is caused by a deficiency in the underlying system
 library function.
 
-** Problems with set-time-zone-rule function
+** Non-US time zones.
 
-The function set-time-zone-rule gives incorrect results for many
+Many non-US time zones are implemented incorrectly.  This is due to
-non-US timezones.  This is due to over-simplistic handling of
+over-simplistic handling of daylight savings switchovers by the
-daylight savings switchovers by the Windows libraries.
+Windows libraries.
 
 ** Files larger than 4GB report wrong size in a 32-bit Windows build
 

lisp/org/org.el

@@ -22674,7 +22674,7 @@ time-range, if possible.
 
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
-`set-time-zone-rule' for a time zone rule."
+the TZ environment variable."
   (format-time-string
    format
    (apply 'encode-time

src/editfns.c

@@ -1969,10 +1969,10 @@ DEFUN ("format-time-string", Fformat_time_string, Sformat_time_string, 1, 3, 0,
        doc: /* Use FORMAT-STRING to format the time TIME, or now if omitted.
 TIME is specified as (HIGH LOW USEC PSEC), as returned by
 `current-time' or `file-attributes'.  The obsolete form (HIGH . LOW)
-is also still accepted.
+is also still accepted.  The optional ZONE is omitted or nil for Emacs
-The optional ZONE is omitted or nil for Emacs local time, t for
+local time, t for Universal Time, `wall' for system wall clock time,
-Universal Time, `wall' for system wall clock time, or a string as in
+or a string as in the TZ environment variable.
-`set-time-zone-rule' for a time zone rule.
+
 The value is a copy of FORMAT-STRING, but with certain constructs replaced
 by text that describes the specified date and time in TIME:
 
@@ -2086,7 +2086,7 @@ as from `current-time' and `file-attributes', or nil to use the
 current time.  The obsolete form (HIGH . LOW) is also still accepted.
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
-`set-time-zone-rule' for a time zone rule.
+the TZ environment variable.
 
 The list has the following nine members: SEC is an integer between 0
 and 60; SEC is 60 for a leap second, which only some operating systems
@@ -2151,9 +2151,9 @@ DEFUN ("encode-time", Fencode_time, Sencode_time, 6, MANY, 0,
 This is the reverse operation of `decode-time', which see.
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
-`set-time-zone-rule' for a time zone rule.  It can also be a list (as
+the TZ environment variable.  It can also be a list (as from
-from `current-time-zone') or an integer (as from `decode-time')
+`current-time-zone') or an integer (as from `decode-time') applied
-applied without consideration for daylight saving time.
+without consideration for daylight saving time.
 
 You can pass more than 7 arguments; then the first six arguments
 are used as SECOND through YEAR, and the *last* argument is used as ZONE.
@@ -2213,7 +2213,7 @@ but this is considered obsolete.
 
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
-`set-time-zone-rule' for a time zone rule.  */)
+the TZ environment variable.  */)
   (Lisp_Object specified_time, Lisp_Object zone)
 {
   time_t value = lisp_seconds_argument (specified_time);
@@ -2290,7 +2290,7 @@ instead of using the current time.  The argument should have the form
 `current-time' and from `file-attributes'.  SPECIFIED-TIME can also
 have the form (HIGH . LOW), but this is considered obsolete.
 Optional second arg ZONE is omitted or nil for the local time zone, or
-a string as in `set-time-zone-rule'.
+a string as in the TZ environment variable.
 
 Some operating systems cannot provide all this information to Emacs;
 in this case, `current-time-zone' returns a list containing nil for
@@ -2331,8 +2331,11 @@ the data it can't find.  */)
 
 DEFUN ("set-time-zone-rule", Fset_time_zone_rule, Sset_time_zone_rule, 1, 1, 0,
        doc: /* Set the Emacs local time zone using TZ, a string specifying a time zone rule.
-If TZ is nil or `wall', use system wall clock time.  If TZ is t, use
+
-Universal Time.  If TZ is an integer, treat it as in `encode-time'.
+If TZ is nil or `wall', use system wall clock time; this differs from
+the usual Emacs convention where nil means current local time.  If TZ
+is t, use Universal Time.  If TZ is an integer, treat it as in
+`encode-time'.
 
 Instead of calling this function, you typically want something else.
 To temporarily use a different time zone rule for just one invocation