public function getLastModified()
    {
        $date = null;

        if ($this->_headers->has('Last-Modified'))
        {
            $value = $this->_headers->get('Last-Modified');
            $date  = new DateTime(date(DATE_RFC2822, strtotime($value)));

            if ($date === false) {
                throw new RuntimeException(sprintf('The Last-Modified HTTP header is not parseable (%s).', $value));
            }
        }

        return $date;
    }