PATCH: gnus-icalendar: fix org-timestamp for events ending at midgnight

[Jan Tatarik <jan.tatarik@gmail.com>]

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Hi,

I just discovered that typical all-day event org timestamp

  <2013-11-19 Tue 00:00>--<2013-11-20 Wed 00:00>

will produce two entries in the org agenda - one for the 19th (expected)
and another one for the 20th (incorrect). It would be similar for any
event ending at midnight.

The enclosed patch fixes the org agenda issue by either omitting the
start/end times entirely (e.g. for all-day events)

  <2013-11-19 Tue>

or by generating end time of 23:59 on the day before

  <2013-11-19 Tue 18:00>--<2013-11-19 Tue 23:59>


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diff --git a/lisp/gnus-icalendar.el b/lisp/gnus-icalendar.el
index a8277635f3e1571722a01cb4b0fe4452ca7d2434..5fbb6e73b3448e744c0579edfa375ac07d6a31d3 100644
--- a/lisp/gnus-icalendar.el
+++ b/lisp/gnus-icalendar.el
@@ -387,14 +387,46 @@ Return nil for non-recurring EVENT."
          (end (gnus-icalendar-event:end-time event))
          (start-date (format-time-string "%Y-%m-%d %a" start))
          (start-time (format-time-string "%H:%M" start))
+         (start-at-midnight (string= start-time "00:00"))
          (end-date (format-time-string "%Y-%m-%d %a" end))
          (end-time (format-time-string "%H:%M" end))
+         (end-at-midnight (string= end-time "00:00"))
+         (start-end-date-diff (/ (float-time (time-subtract
+                                        (date-to-time end-date)
+                                        (date-to-time start-date)))
+                                 86400))
          (org-repeat (gnus-icalendar-event:org-repeat event))
-         (repeat (if org-repeat (concat " " org-repeat) "")))
-
-    (if (equal start-date end-date)
-        (format "<%s %s-%s%s>" start-date start-time end-time repeat)
-      (format "<%s %s>--<%s %s>" start-date start-time end-date end-time))))
+         (repeat (if org-repeat (concat " " org-repeat) ""))
+         (time-1-day '(0 86400)))
+
+    ;; NOTE: special care is needed with appointments ending at midnight
+    ;; (typically all-day events): the end time has to be changed to 23:59 to
+    ;; prevent org agenda showing the event on one additional day
+    (cond
+     ;; start/end midnight
+     ;; A 0:0 - A+1 0:0 -> A
+     ;; A 0:0 - A+n 0:0 -> A - A+n-1
+     ((and start-at-midnight end-at-midnight) (if (> start-end-date-diff 1)
+                                                  (let ((end-ts (format-time-string "%Y-%m-%d %a" (time-subtract end time-1-day))))
+                                                    (format "<%s>--<%s>" start-date end-ts))
+                                                (format "<%s%s>" start-date repeat)))
+     ;; end midnight
+     ;; A .:. - A+1 0:0 -> A .:.-23:59
+     ;; A .:. - A+n 0:0 -> A .:. - A_n-1
+     (end-at-midnight (if (= start-end-date-diff 1)
+                          (format "<%s %s-23:59%s>" start-date start-time repeat)
+                        (let ((end-ts (format-time-string "%Y-%m-%d %a" (time-subtract end time-1-day))))
+                          (format "<%s %s>--<%s>" start-date start-time end-ts))))
+     ;; start midnight
+     ;; A 0:0 - A .:. -> A 0:0-.:. (default 1)
+     ;; A 0:0 - A+n .:. -> A - A+n .:.
+     ((and start-at-midnight
+           (plusp start-end-date-diff)) (format "<%s>--<%s %s>" start-date end-date end-time))
+     ;; default
+     ;; A .:. - A .:. -> A .:.-.:.
+     ;; A .:. - B .:.
+     ((zerop start-end-date-diff) (format "<%s %s-%s%s>" start-date start-time end-time repeat))
+     (t (format "<%s %s>--<%s %s>" start-date start-time end-date end-time)))))
 
 (defun gnus-icalendar--format-summary-line (summary &optional location)
   (if location