diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index 03cdb8bf..f3316dc6 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -809,6 +809,8 @@ class calendar extends rcube_plugin
           $action = 'import';
         }
         else if (in_array($status, array('ACCEPTED','TENTATIVE','DECLINED'))) {
+          if (is_numeric($event['changed']))
+            $event['changed'] = new DateTime('@'.$event['changed']);
           $html = html::div('rsvp-status ' . strtolower($status), $this->gettext('youhave'.strtolower($status)));
           if ($existing['sequence'] >= $event['sequence'] || (!$event['sequence'] && $existing['changed'] && $existing['changed'] >= $event['changed'])) {
             $action = '';  // nothing to do here
@@ -1199,6 +1201,7 @@ 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'),
       'title'       => strval($event['title']),
       'description' => strval($event['description']),
       'location'    => strval($event['location']),
@@ -1854,7 +1857,7 @@ class calendar extends rcube_plugin
           $buttons .= html::div(array('id' => 'import-'.$dom_id, 'style' => 'display:none'), $import_button);
           $buttons_pre = html::div(array('id' => 'loading-'.$dom_id, 'class' => 'rsvp-status loading'), $this->gettext('loading'));
           
-          $this->rc->output->add_script('rcube_calendar.fetch_event_rsvp_status(' . json_serialize(array('uid' => $event['uid'], 'changed' => $event['changed'], 'sequence' => intval($event['sequence']), 'fallback' => $status)) . ')', 'docready');
+          $this->rc->output->add_script('rcube_calendar.fetch_event_rsvp_status(' . json_serialize(array('uid' => $event['uid'], 'changed' => $event['changed']->format('U'), 'sequence' => intval($event['sequence']), 'fallback' => $status)) . ')', 'docready');
         }
         else if ($this->ical->method == 'CANCEL') {
           $title = $this->gettext('itipcancellation');
@@ -1878,7 +1881,7 @@ class calendar extends rcube_plugin
           $buttons .= html::div(array('id' => 'import-'.$dom_id, 'style' => 'display:none'), $button_import);
           $buttons_pre = html::div(array('id' => 'loading-'.$dom_id, 'class' => 'rsvp-status loading'), $this->gettext('loading'));
           
-          $this->rc->output->add_script('rcube_calendar.fetch_event_rsvp_status(' . json_serialize(array('uid' => $event['uid'], 'changed' => $event['changed'], 'sequence' => intval($event['sequence']), 'fallback' => 'CANCELLED')) . ')', 'docready');
+          $this->rc->output->add_script('rcube_calendar.fetch_event_rsvp_status(' . json_serialize(array('uid' => $event['uid'], 'changed' => $event['changed']->format('U'), 'sequence' => intval($event['sequence']), 'fallback' => 'CANCELLED')) . ')', 'docready');
         }
         else {
           $buttons = html::tag('input', array(
diff --git a/plugins/calendar/drivers/database/database_driver.php b/plugins/calendar/drivers/database/database_driver.php
index c8c54b6b..9388be32 100644
--- a/plugins/calendar/drivers/database/database_driver.php
+++ b/plugins/calendar/drivers/database/database_driver.php
@@ -774,7 +774,7 @@ class database_driver extends calendar_driver
     $event['start'] = new DateTime($event['start']);
     $event['end'] = new DateTime($event['end']);
     $event['allday'] = intval($event['all_day']);
-    $event['changed'] = strtotime($event['changed']);
+    $event['changed'] = new DateTime($event['changed']);
     $event['free_busy'] = $free_busy_map[$event['free_busy']];
     $event['calendar'] = $event['calendar_id'];
     $event['recurrence_id'] = intval($event['recurrence_id']);
diff --git a/plugins/calendar/lib/calendar_itip.php b/plugins/calendar/lib/calendar_itip.php
index 51a1c92b..dc9a6922 100644
--- a/plugins/calendar/lib/calendar_itip.php
+++ b/plugins/calendar/lib/calendar_itip.php
@@ -238,7 +238,7 @@ class calendar_itip
           $this->sender = $attendee;
         }
       }
-      $invitation['event']['changed'] = time();
+      $invitation['event']['changed'] = new DateTime();
       
       // send iTIP REPLY message to organizer
       if ($organizer) {
diff --git a/plugins/libcalendaring/libvcalendar.php b/plugins/libcalendaring/libvcalendar.php
index be6f93db..a9ba04c4 100644
--- a/plugins/libcalendaring/libvcalendar.php
+++ b/plugins/libcalendaring/libvcalendar.php
@@ -141,7 +141,7 @@ class libvcalendar
   {
     $event = array(
       'uid' => $ve->getAttributeDefault('UID'),
-      'changed' => $ve->getAttributeDefault('DTSTAMP', 0),
+      'changed' => $this->_date2time($ve->getAttributeDefault('DTSTAMP', 0)),
       'title' => $ve->getAttributeDefault('SUMMARY'),
       'start' => $this->_date2time($ve->getAttribute('DTSTART')),
       'end' => $this->_date2time($ve->getAttribute('DTEND')),