diff --git a/htdocs/core/lib/xcal.lib.php b/htdocs/core/lib/xcal.lib.php
index f0edbd6f877..a4ddb68dbd3 100644
--- a/htdocs/core/lib/xcal.lib.php
+++ b/htdocs/core/lib/xcal.lib.php
@@ -180,7 +180,9 @@ function build_calfile($format, $title, $desc, $events_array, $outputfile)
 
                     if (! empty($location)) fwrite($calfileh, "LOCATION:".$encoding.$location."\n");
 					if ($fulldayevent) fwrite($calfileh, "X-FUNAMBOL-ALLDAY:1\n");
-                    if ($fulldayevent) fwrite($calfileh, "X-MICROSOFT-CDO-ALLDAYEVENT:1\n");
+					
+					// see https://docs.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxcical/0f262da6-c5fd-459e-9f18-145eba86b5d2
+                    if ($fulldayevent) fwrite($calfileh, "X-MICROSOFT-CDO-ALLDAYEVENT:TRUE\n");
 
 					// Date must be GMT dates
 					// Current date