fixed Event\Adapter\ORM in Timestampable

lib/Gedmo/Timestampable/Mapping/Event/Adapter/ORM.php

@@ -22,6 +22,7 @@ final class ORM extends BaseAdapterORM implements TimestampableAdapter
      */
     public function getDateValue($meta, $field)
     {
+        $mapping = $meta->getFieldMapping($field);
         if (isset($mapping['type']) && $mapping['type'] == 'zenddate') {
             return new \Zend_Date();
         }