diff --git a/plugins/libkolab/lib/kolab_storage_cache.php b/plugins/libkolab/lib/kolab_storage_cache.php
index b5e56831..03367c4c 100644
--- a/plugins/libkolab/lib/kolab_storage_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_cache.php
@@ -1456,7 +1456,7 @@ class kolab_storage_cache
             $dt->setTimeZone($this->server_timezone);
             return $dt->format(self::DB_DATE_FORMAT);
         } elseif ($datetime) {
-            return date(self::DB_DATE_FORMAT, $datetime);
+            return date(self::DB_DATE_FORMAT, (int) $datetime);
         }
     }
 }