public function _getTimestampOrDuration()
    {
        if (null === $this->_buffer) {
            return [null, null];
        }

        list($now, $then) = $this->_initNowThen();

        $this->_mode = null;
        // apply buffered methods
        foreach ($this->_buffer as $call) {
            $then = $this->_apply($call[0], $call[1], $now, $then);
        }
        // has user used a method by timestamp?
        $isTimestamp = null !== $this->_mode;
        //return
        if ($isTimestamp) {
            return [$then->timestamp, null];
        }

        return [null, $now->diffInSeconds($then)];
    }