From 4df9db3eeb3b048579768050ae7bb413fefe9557 Mon Sep 17 00:00:00 2001
From: Yuchen Pei <hi@ypei.me>
Date: Wed, 25 Aug 2021 13:37:20 +1000
Subject: [PATCH] Adding diary-offset to diary-lib.el.

A new diary sexp that offsets another diary sexp (Bug#50195).

* doc/emacs/calendar.texi: Document the change.
* lisp/calendar/diary-lib.el (diary-offset):
* test/lisp/calendar/icalendar-tests.el: Add a test.
---
 doc/emacs/calendar.texi               | 14 ++++++++++++++
 etc/NEWS                              |  8 ++++++++
 lisp/calendar/diary-lib.el            | 11 +++++++++++
 test/lisp/calendar/icalendar-tests.el |  7 +++++++
 4 files changed, 40 insertions(+)

diff --git a/doc/emacs/calendar.texi b/doc/emacs/calendar.texi
index 3750e78e70..18de721e28 100644
--- a/doc/emacs/calendar.texi
+++ b/doc/emacs/calendar.texi
@@ -1363,6 +1363,20 @@ Special Diary Entries
 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
diff --git a/etc/NEWS b/etc/NEWS
index 07a78216b8..52774b3463 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2800,6 +2800,14 @@ 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
 
 ---
diff --git a/lisp/calendar/diary-lib.el b/lisp/calendar/diary-lib.el
index f57fe26058..f71392b97f 100644
--- a/lisp/calendar/diary-lib.el
+++ b/lisp/calendar/diary-lib.el
@@ -2014,6 +2014,17 @@ diary-cyclic
     (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))
+  (or (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))
diff --git a/test/lisp/calendar/icalendar-tests.el b/test/lisp/calendar/icalendar-tests.el
index 6973f7e5c9..de2a891758 100644
--- a/test/lisp/calendar/icalendar-tests.el
+++ b/test/lisp/calendar/icalendar-tests.el
@@ -1442,6 +1442,13 @@ icalendar-real-world
 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
-- 
2.33.0