Add diary-offset to diary-lib.el

* doc/emacs/calendar.texi: Document the change.
* lisp/calendar/diary-lib.el (diary-offset):
* test/lisp/calendar/icalendar-tests.el: Add a test (bug#50195).

doc/emacs/calendar.texi

@@ -1363,6 +1363,20 @@ the 11 above to @samp{'(1 2 3)} and have the entry apply to the last
 Thursday of January, February, and March.  If the month is @code{t}, the
 entry applies to all months of the year.
 
+@findex diary-offset
+@example
+%%(diary-offset '(diary-float t 3 4) 2) Monthly committee meeting
+@end example
+
+@noindent
+This entry applies to the Saturday after the third Thursday of each
+month.  The 2 specifies number of days after when the sexp
+@w{@code{'(diary-float t 3 4)}} would evaluate to @code{t}.  This is
+useful when for example your organization has a committee meeting two
+days after every monthly meeting which takes place on the third
+Thursday, or if you would like to attend a virtual meeting scheduled
+in a different timezone causing a difference in the date.
+
   Each of the standard sexp diary entries takes an optional parameter
 specifying the name of a face or a single-character string to use when
 marking the entry in the calendar.  Most generally, sexp diary entries

etc/NEWS

@@ -2814,6 +2814,14 @@ never be narrower than 19 characters.
 'gomoku-move-sw' and 'gomoku-move-ne' now work correctly, and
 horizontal movements now stop at the edge of the board.
 
++++
+*** New diary sexp 'diary-offset'.
+It offsets another diary sexp by a number of days.  This is useful
+when for example your organization has a committee meeting two days
+after every monthly meeting which takes place on the third Thursday,
+or if you would like to attend a virtual meeting scheduled in a
+different timezone causing a difference in the date.
+
 ** xterm-mouse mode
 
 ---

lisp/calendar/diary-lib.el

@@ -2014,6 +2014,17 @@ string to use when highlighting the day in the calendar."
     (and (>= diff 0) (zerop (% diff n))
          (cons mark (format entry cycle (diary-ordinal-suffix cycle))))))
 
+;; To be called from diary-sexp-entry, where DATE, ENTRY are bound.
+(defun diary-offset (sexp days)
+  "Offsetted diary entry.  Offsets SEXP by DAYS days.
+Entry applies if the date is DAYS days after another diary-sexp SEXP."
+  (with-no-warnings (defvar date))
+  (unless (integerp days)
+    (user-error "Days must be an integer"))
+  (let ((date (calendar-gregorian-from-absolute
+	       (- (calendar-absolute-from-gregorian date) days))))
+    (eval sexp)))
+
 (defun diary-day-of-year ()
   "Day of year and number of days remaining in the year of date diary entry."
   (with-no-warnings (defvar date))

test/lisp/calendar/icalendar-tests.el

@@ -1442,6 +1442,13 @@ RRULE:FREQ=YEARLY;INTERVAL=1;BYMONTH=09;BYMONTHDAY=21
 SUMMARY:ff birthday (%d years old)")
 
 
+  (icalendar-tests--test-export
+   nil
+   nil
+   "%%(diary-offset '(diary-float t 3 4) 1) asdf"
+   nil)
+
+
   ;; FIXME!
 
   ;; export 2004-10-28 monthly, weekly entries