private function _validateTime()
    {
        $t = $this->_config->evaluationTime();
        $validity = $this->_ac->acinfo()->validityPeriod();
        if ($validity->notBeforeTime()->diff($t)->invert) {
            throw new ACValidationException("Validity period has not started.");
        }
        if ($t->diff($validity->notAfterTime())->invert) {
            throw new ACValidationException("Attribute certificate has expired.");
        }
    }