protected function generateExpiresHeader()
    {
        $maxAge = $this->getDirective('max-age');
        if ($maxAge === false) {
            return null;
        }

        // Add now to max-age to generate expires
        $expires = DBDatetime::now()->getTimestamp() + $maxAge;
        return gmdate('D, d M Y H:i:s', $expires) . ' GMT';
    }