public function getAge()
    {
        if ($this->headers->has('Age')) {
            return $this->headers->get('Age');
        } else {
            $dateTimestamp = $this->headers->get('Date')->getTimestamp();
            $nowTimestamp = $this->now->getTimestamp();
            return ($nowTimestamp > $dateTimestamp) ? ($nowTimestamp - $dateTimestamp) : 0;
        }
    }