Index: entity/includes/entity.property.inc
===================================================================
--- entity/includes/entity.property.inc	(revision 32)
+++ entity/includes/entity.property.inc	(working copy)
@@ -210,6 +210,7 @@
     case 'unknown':
       return TRUE;
     case 'date':
+      return is_numeric($data) || preg_match('!^[0-9]{4}-[0-9]{2}-[0-9]{2}[\sT][0-9]{2}:[0-9]{2}:[0-9]{2}$!', $data);
     case 'duration':
     case 'integer':
       return is_numeric($data) && strpos($data, '.') === FALSE;