private function timestampHasExpired($timestamp) {
        $year   = substr($timestamp, 0, 4);
        $month  = substr($timestamp, 5, 2);
        $day    = substr($timestamp, 8, 2);
        $hour   = substr($timestamp, 11, 2);
        $minute = substr($timestamp, 14, 2);
        $second = substr($timestamp, 17, 2);

        $timestamp = gmmktime($hour, $minute, $second, $month, $day, $year);

        $diff = time() - $timestamp;

        if ($diff > $this->maxDiff || $diff < -$this->maxDiff) {
            return true;
        }

        return false;
    }