public function expires($time)
    {
        if (!empty($time) && is_string($time) && !$timestamp = strtotime($time)) {
            throw new \Ergo\Routing\Exception("Invalid expiry time: $timestamp");
        } else if (is_numeric($time)) {
            $timestamp = $time;
        } else if ($time == false) {
            return $this;
        }

        $this->addHeader('Expires', date('r', $timestamp));

        return $this;
    }