diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index 5c866da7..3b8728a3 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -1205,7 +1205,8 @@ class calendar extends rcube_plugin
       '_id'   => $event['calendar'] . ':' . $event['id'],  // unique identifier for fullcalendar
       'start' => $this->lib->adjust_timezone($event['start'])->format('c'),
       'end'   => $this->lib->adjust_timezone($event['end'])->format('c'),
-      'changed' => $this->lib->adjust_timezone($event['changed'])->format('c'),
+      // 'changed' might be empty for event recurrences (Bug #2185)
+      'changed' => $event['changed'] ? $this->lib->adjust_timezone($event['changed'])->format('c') : null,
       'title'       => strval($event['title']),
       'description' => strval($event['description']),
       'location'    => strval($event['location']),
diff --git a/plugins/libcalendaring/libcalendaring.php b/plugins/libcalendaring/libcalendaring.php
index 31b731da..608a5277 100644
--- a/plugins/libcalendaring/libcalendaring.php
+++ b/plugins/libcalendaring/libcalendaring.php
@@ -120,7 +120,10 @@ class libcalendaring extends rcube_plugin
         else if (is_string($dt))
             $dt = new DateTime($dt);
 
-        $dt->setTimezone($this->timezone);
+        if ($dt instanceof DateTime) {
+            $dt->setTimezone($this->timezone);
+        }
+
         return $dt;
     }