public function validateTimestamps()
    {
        $rootNode = $this->_document;
        $timestampNodes = $rootNode->getElementsByTagName('Conditions');
        for ($i = 0; $i < $timestampNodes->length; $i++) {
            $nbAttribute = $timestampNodes->item($i)->attributes->getNamedItem("NotBefore");
            $naAttribute = $timestampNodes->item($i)->attributes->getNamedItem("NotOnOrAfter");
            if ($nbAttribute && strtotime($nbAttribute->textContent) > time()) {
                return false;
            }
            if ($naAttribute && strtotime($naAttribute->textContent) <= time()) {
                return false;
            }
        }
        return true;
    }