public function timestamp(DateTime $ts = null, $refreshTimestamp = false)
    {
        if ($ts) {
            if ($this->_config['refreshTimestamp']) {
                $this->_config['refreshTimestamp'] = false;
            }
            $this->_ts = new Time($ts);
        } elseif ($this->_ts === null || $refreshTimestamp) {
            $this->_ts = new Time();
        }

        return $this->_ts;
    }