diff --git a/phpgwapi/inc/horde/Horde/iCalendar.php b/phpgwapi/inc/horde/Horde/iCalendar.php
index caa1c05add..efd13629eb 100644
--- a/phpgwapi/inc/horde/Horde/iCalendar.php
+++ b/phpgwapi/inc/horde/Horde/iCalendar.php
@@ -1622,6 +1622,11 @@ class Horde_iCalendar {
                 $duration .= $value . 'S';
             }
         }
+		// duration without time ("P") is NOT valid, append 0 minutes ("T0M")
+		elseif ($duration === 'P')
+		{
+			$duration .= 'T0M';
+		}
 
         return $duration;
     }