private function _checkValidity(Certificate $cert)
    {
        $refdt = $this->_config->dateTime();
        $validity = $cert->tbsCertificate()->validity();
        if ($validity->notBefore()
            ->dateTime()
            ->diff($refdt)->invert) {
            throw new PathValidationException(
                "Certificate validity period has not started.");
        }
        if ($refdt->diff($validity->notAfter()
            ->dateTime())->invert) {
            throw new PathValidationException("Certificate has expired.");
        }
    }