public function getSessionNotOnOrAfter()
    {
        $notOnOrAfter = null;
        $entries = $this->_queryAssertion('/saml:AuthnStatement[@SessionNotOnOrAfter]');
        if ($entries->length !== 0) {
            $notOnOrAfter = OneLogin_Saml2_Utils::parseSAML2Time($entries->item(0)->getAttribute('SessionNotOnOrAfter'));
        }
        return $notOnOrAfter;
    }