diff --git a/calendar/inc/class.calendar_ical.inc.php b/calendar/inc/class.calendar_ical.inc.php
index ca0a98df0c..7eb2c694cc 100644
--- a/calendar/inc/class.calendar_ical.inc.php
+++ b/calendar/inc/class.calendar_ical.inc.php
@@ -848,7 +848,11 @@ class calendar_ical extends calendar_boupdate
 			{
 				$attributes['CREATED'] = $event['created'] ? $event['created'] : $event['modified'];
 			}
-			if ($event['modified'])
+			if ($event['max_user_modified'])
+			{
+				$attributes['LAST-MODIFIED'] = max($event['modified'], $event['max_user_modified']);
+			}
+			elseif ($event['modified'])
 			{
 				$attributes['LAST-MODIFIED'] = $event['modified'];
 			}
diff --git a/calendar/inc/class.calendar_so.inc.php b/calendar/inc/class.calendar_so.inc.php
index 19e2275e81..c5c05a06fd 100644
--- a/calendar/inc/class.calendar_so.inc.php
+++ b/calendar/inc/class.calendar_so.inc.php
@@ -241,6 +241,11 @@ class calendar_so
 
 			$events[$row['cal_id']]['participants'][$uid] = $status;
 			$events[$row['cal_id']]['participant_types'][$row['cal_user_type']][$row['cal_user_id']] = $status;
+
+			if (($modified = $this->db->from_timestamp($row['cal_user_modified'])) > $events[$row['cal_id']]['max_user_modified'])
+			{
+				$events[$row['cal_id']]['max_user_modified'] = $modified;
+			}
 		}
 
 		// custom fields