diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index 91c7ae55..1dba9c8f 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -1141,11 +1141,11 @@ class kolab_driver extends calendar_driver
         // use start date from master but try to be smart on time or duration changes
         $old_start_date = $old['start']->format('Y-m-d');
         $old_start_time = $old['allday'] ? '' : $old['start']->format('H:i');
-        $old_duration   = $old['allday'] ? 0 : $old['end']->format('U') - $old['start']->format('U');
+        $old_duration   = self::event_duration($old['start'], $old['end'], $old['allday']);
 
         $new_start_date = $event['start']->format('Y-m-d');
         $new_start_time = $event['allday'] ? '' : $event['start']->format('H:i');
-        $new_duration   = $event['allday'] ? 0 : $event['end']->format('U') - $event['start']->format('U');
+        $new_duration   = self::event_duration($event['start'], $event['end'], $event['allday']);
 
         $diff = $old_start_date != $new_start_date || $old_start_time != $new_start_time || $old_duration != $new_duration;
         $date_shift = $old['start']->diff($event['start']);
@@ -1154,7 +1154,7 @@ class kolab_driver extends calendar_driver
         if ($diff && ($old_start_date == $new_start_date || $old_duration == $new_duration)) {
           $event['start'] = $master['start']->add($date_shift);
           $event['end'] = clone $event['start'];
-          $event['end']->add(new DateInterval('PT'.$new_duration.'S'));
+          $event['end']->add(new DateInterval($new_duration));
 
           // remove fixed weekday, will be re-set to the new weekday in kolab_calendar::update_event()
           if ($old_start_date != $new_start_date) {
@@ -1230,6 +1230,19 @@ class kolab_driver extends calendar_driver
     return $success;
   }
 
+  /**
+   * Calculate event duration, returns string in DateInterval format
+   */
+  protected static function event_duration($start, $end, $allday = false)
+  {
+    if ($allday) {
+      $diff = $start->diff($end);
+      return 'P' . $diff->days . 'D';
+    }
+
+    return 'PT' . ($end->format('U') - $start->format('U')) . 'S';
+  }
+
   /**
    * Determine whether the current change affects scheduling and reset attendee status accordingly
    */