diff --git a/lisp/gnus/gnus-icalendar.el b/lisp/gnus/gnus-icalendar.el
index 4faef06..82a649f 100644
--- a/lisp/gnus/gnus-icalendar.el
+++ b/lisp/gnus/gnus-icalendar.el
@@ -401,8 +401,8 @@ gnus-icalendar-event:org-timestamp
          (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)))
+                                        (org-time-string-to-time end-date)
+                                        (org-time-string-to-time start-date)))
                                  86400))
          (org-repeat (gnus-icalendar-event:org-repeat event))
          (repeat (if org-repeat (concat " " org-repeat) ""))