public static function cleanupExpiredSubscription( $time )
    {
        //1. find the subscription which is disabled and whose time is too long
        $startTime = time() - $time;
        $db = eZDB::instance();
        $selectSql = "SELECT id FROM ezcomment_subscription WHERE subscription_time < $startTime AND enabled = 0";
        $result = $db->arrayQuery( $selectSql );
        if ( is_array( $result ) && count( $result ) > 0 )
        {
            //2. clean up the subscription
            $deleteSql = "DELETE FROM ezcomment_subscription WHERE subscription_time < $startTime AND enabled = 0";
            $db->query( $deleteSql );
            return $result;
        }
        else
        {
            return null;
        }
    }