private function isModified($timestamp)
    {
        $target = 'If-Modified-Since';

        if ($this->headerBag->hasRequestHeader($target)) {
            $sinceTimestamp = strtotime($this->headerBag->getRequestHeader($target));

            if ($sinceTimestamp != false && $timestamp <= $sinceTimestamp) {
                return false;
            }
        }

        // By default
        return true;
    }