org-manual, org-guide: Improve timestamp documentation

* doc/org-manual.org (Dates and Times): Explain the purpose of
timestamps in the opening paragraphs.
(Timestamps): Mention that time range.  Clarify that an entry may
contain multiple timestamps, providing an example.  Explain formal
time range meaning, with example.  Add cindex entry "time range".  Add
date range example.
* doc/org-guide.org: Syncronoize changes with org-manual, adding some
missing pieces.
(Timestamps): Remove useless cindex entries (org-guide does not
produce index).

Co-authored-by: Ihor Radchenko <yantar92@posteo.net>
Link: https://orgmode.org/list/SJ0PR03MB5455693CA8266256D46BA594A259A@SJ0PR03MB5455.namprd03.prod.outlook.com

TINYCHANGE

doc/org-guide.org

@@ -1081,7 +1081,15 @@ Tags and Properties]].
 
 To assist project planning, TODO items can be labeled with a date
 and/or a time.  The specially formatted string carrying the date and
-time information is called a /timestamp/ in Org mode.
+time information is called a /timestamp/ in Org mode.  This may be
+a little confusing because timestamp is often used as indicating when
+something was created or last changed.  However, in Org mode this term
+is used in a much wider sense.
+
+Timestamps can be used to plan appointments, schedule tasks, set
+deadlines, track time, and more.  The following sections describe
+the timestamp format and tooling that Org mode provides for common
+use cases dealing with time and time intervals.
 
 ** Timestamps
 :PROPERTIES:
@@ -1099,12 +1107,16 @@ the agenda (see [[*The Weekly/Daily Agenda]]).  We distinguish:
 
   A simple timestamp just assigns a date/time to an item.  This is
   just like writing down an appointment or event in a paper agenda.
+  There can be multiple timestamps in an item.
 
   #+begin_example
   ,* Meet Peter at the movies
     <2006-11-01 Wed 19:15>
   ,* Discussion on climate change
     <2006-11-02 Thu 20:00-22:00>
+  ,* My days off
+    <2006-11-03 Fri>
+    <2006-11-06 Mon>
   #+end_example
 
 - Timestamp with repeater interval ::
@@ -1121,8 +1133,6 @@ the agenda (see [[*The Weekly/Daily Agenda]]).  We distinguish:
 
 - Diary-style expression entries ::
 
-  #+cindex: diary style timestamps
-  #+cindex: sexp timestamps
   For more complex date specifications, Org mode supports using the
   special expression diary entries implemented in the Emacs Calendar
   package.  For example, with optional time:
@@ -1132,13 +1142,28 @@ the agenda (see [[*The Weekly/Daily Agenda]]).  We distinguish:
     <%%(diary-float t 4 2)>
   #+end_example
 
+- Time range ::
+
+  Time range is a timestamp having two time units connected by =-=
+
+  #+begin_example
+  ,* Discussion on climate change
+    <2006-11-02 Thu 10:00-12:00>
+  #+end_example
+
 - Time/Date range ::
 
-  Two timestamps connected by =--= denote a range.
+  Two timestamps connected by =--= denote a range.  In the agenda, the
+  headline is shown on the first and last day of the range, and on any
+  dates that are displayed and fall in the range.  The first example
+  specifies just the dates of the range while the second example
+  specifies a time range for each date.
 
   #+begin_example
   ,** Meeting in Amsterdam
      <2004-08-23 Mon>--<2004-08-26 Thu>
+  ,** This weeks committee meetings
+     <2004-08-23 Mon 10:00-11:00>--<2004-08-26 Thu 10:00-11:00>
   #+end_example
 
 - Inactive timestamp ::

doc/org-manual.org

@@ -5987,6 +5987,11 @@ a little confusing because timestamp is often used as indicating when
 something was created or last changed.  However, in Org mode this term
 is used in a much wider sense.
 
+Timestamps can be used to plan appointments, schedule tasks, set
+deadlines, track time, and more.  The following sections describe
+the timestamp format and tooling that Org mode provides for common
+use cases dealing with time and time intervals.
+
 ** Timestamps
 :PROPERTIES:
 :DESCRIPTION: Assigning a time to a tree entry.
@@ -5997,12 +6002,12 @@ is used in a much wider sense.
 #+cindex: deadlines
 #+cindex: scheduling
 
-A timestamp is a specification of a date (possibly with a time) in a
+A timestamp is a specification of a date---possibly with a time or
-special format, either =<2003-09-16 Tue>= or
+time range---in a special format, either =<2003-09-16 Tue>= or
-=<2003-09-16 Tue 09:39>=[fn:19].  A timestamp can appear anywhere in
+=<2003-09-16 Tue 09:39>= or =<2003-09-16 Tue 12:00-12:30>=[fn:19].
-the headline or body of an Org tree entry.  Its presence causes
+A timestamp can appear anywhere in the headline or body of an Org tree
-entries to be shown on specific dates in the agenda (see [[*Weekly/daily
+entry.  Its presence causes entries to be shown on specific dates in
-agenda]]).  We distinguish:
+the agenda (see [[*Weekly/daily agenda]]).  We distinguish:
 
 - Plain timestamp; Event; Appointment ::
 
@@ -6011,13 +6016,17 @@ agenda]]).  We distinguish:
   A simple timestamp just assigns a date/time to an item.  This is
   just like writing down an appointment or event in a paper agenda.
   In the agenda display, the headline of an entry associated with
-  a plain timestamp is shown exactly on that date.
+  a plain timestamp is shown exactly on that date.  There can be
+  multiple timestamps in an item.
 
   #+begin_example
   ,* Meet Peter at the movies
     <2006-11-01 Wed 19:15>
   ,* Discussion on climate change
-    <2006-11-02 Thu>
+    <2006-11-02 Thu 10:00-12:00>
+  ,* My days off
+    <2006-11-03 Fri>
+    <2006-11-06 Mon>
   #+end_example
 
 - Timestamp with repeater interval ::
@@ -6053,24 +6062,32 @@ agenda]]).  We distinguish:
     <%%(diary-float t 4 2)>
   #+end_example
 
+- Time range ::
+  #+cindex: time range
+
+  Time range is a timestamp having two time units connected by =-=
+
+  #+begin_example
+,* Discussion on climate change
+   <2006-11-02 Thu 10:00-12:00>
+  #+end_example
+
 - Time/Date range ::
 
+  #+cindex: time range
   #+cindex: timerange
   #+cindex: date range
   Two timestamps connected by =--= denote a range.  In the agenda, the
   headline is shown on the first and last day of the range, and on any
-  dates that are displayed and fall in the range.  Here is an example:
+  dates that are displayed and fall in the range.  The first example
+  specifies just the dates of the range while the second example
+  specifies a time range for each date.
 
   #+begin_example
   ,** Meeting in Amsterdam
      <2004-08-23 Mon>--<2004-08-26 Thu>
-  #+end_example
+  ,** This weeks committee meetings
-
+     <2004-08-23 Mon 10:00-11:00>--<2004-08-26 Thu 10:00-11:00>
-  Timerange is a timestamp consisting of two time units connected by =-=
-
-  #+begin_example
-  ,* Discussion on climate change
-     <2006-11-02 Thu 10:00-12:00>
   #+end_example
 
 - Inactive timestamp ::