--- ox-icalendar.el.orig	2022-01-02 13:38:17.586027624 +0100
+++ ox-icalendar.el	2022-01-02 14:12:10.446157056 +0100
@@ -265,6 +265,19 @@
 	  (const :tag "Universal time" ":%Y%m%dT%H%M%SZ")
 	  (string :tag "Explicit format")))
 
+(defcustom org-icalendar-ttl "PT1H"
+  "The time to life for the exported calendar.
+Subscribing clients to the exported ics file can derive the time interval
+to read the file again from the server. One example of such a client is
+the nextcloud calendar, which respects the setting of
+X-PUBLISHED-TTL, i.e. X-PUBLISHED-TTL:PT1H .
+
+See https://icalendar.org/iCalendar-RFC-5545/3-8-2-5-duration.html
+for a complete description of possiblee values of this option. I.e.
+PT1H stands for 1 hour, PT0H27M34S for 0 hours, 27 minutes and 34 seconds."
+  :group 'org-export-icalendar
+  :type 'string)
+
 (defvar org-icalendar-after-save-hook nil
   "Hook run after an iCalendar file has been saved.
 This hook is run with the name of the file as argument.  A good
@@ -300,7 +313,8 @@
     (:icalendar-store-UID nil nil org-icalendar-store-UID)
     (:icalendar-timezone nil nil org-icalendar-timezone)
     (:icalendar-use-deadline nil nil org-icalendar-use-deadline)
-    (:icalendar-use-scheduled nil nil org-icalendar-use-scheduled))
+    (:icalendar-use-scheduled nil nil org-icalendar-use-scheduled)
+    (:icalendar-ttl nil nil org-icalendar-ttl))
   :filters-alist
   '((:filter-headline . org-icalendar-clear-blank-lines))
   :menu-entry
@@ -828,24 +842,29 @@
      (cadr (current-time-zone)))
    ;; Description.
    (org-export-data (plist-get info :title) info)
+   ;; TTL
+   org-icalendar-ttl
    contents))
 
-(defun org-icalendar--vcalendar (name owner tz description contents)
+(defun org-icalendar--vcalendar (name owner tz description ttl contents)
   "Create a VCALENDAR component.
-NAME, OWNER, TZ, DESCRIPTION and CONTENTS are all strings giving,
+NAME, OWNER, TZ, DESCRIPTION, TTL and CONTENTS are all strings giving,
 respectively, the name of the calendar, its owner, the timezone
-used, a short description and the other components included."
+used, a short description, the time-to-live resp. refresh period and 
+the other components included."
   (concat (format "BEGIN:VCALENDAR
 VERSION:2.0
 X-WR-CALNAME:%s
 PRODID:-//%s//Emacs with Org mode//EN
 X-WR-TIMEZONE:%s
 X-WR-CALDESC:%s
+X-PUBLISHED-TTL:%s
 CALSCALE:GREGORIAN\n"
 		  (org-icalendar-cleanup-string name)
 		  (org-icalendar-cleanup-string owner)
 		  (org-icalendar-cleanup-string tz)
-		  (org-icalendar-cleanup-string description))
+		  (org-icalendar-cleanup-string description)
+		  (org-icalendar-cleanup-string ttl))
 	  contents
 	  "END:VCALENDAR\n"))
 
@@ -974,6 +993,7 @@
 	user-full-name
 	(or (org-string-nw-p org-icalendar-timezone) (cadr (current-time-zone)))
 	org-icalendar-combined-description
+	org-icalendar-ttl
 	contents)))
     (run-hook-with-args 'org-icalendar-after-save-hook file)))
 
@@ -998,6 +1018,8 @@
 		  (cadr (current-time-zone)))
 	      ;; Description.
 	      org-icalendar-combined-description
+	      ;; TTL (Refresh period)
+	      org-icalendar-ttl
 	      ;; Contents.
 	      (concat
 	       ;; Agenda contents.