public function urlQuerystring($url)
    {
        $querystring = $this->querystring();
        if (isset($this->timestamp) && !Utils::contains($querystring, $this->timestamp)) {
            $querystring = empty($querystring) ? ('?' . $this->timestamp) : ($querystring . '&' . $this->timestamp);
        }

        return ltrim($url . $querystring . $this->urlHash(), '/');
    }