private function updateDiffTimestamp()
    {
        $diffTimestamp = 0;
        $lastTimeStamp = 0;
        $lastActivity = 0;
        if (isset($this->data[DeliveryMonitoringService::LAST_PAUSE_TIMESTAMP])) {
            $lastTimeStamp = $this->data[DeliveryMonitoringService::LAST_PAUSE_TIMESTAMP];
        }

        if (isset($this->data[DeliveryMonitoringService::LAST_TEST_TAKER_ACTIVITY])) {
            $lastActivity = $this->data[DeliveryMonitoringService::LAST_TEST_TAKER_ACTIVITY];
        }

        if ($lastTimeStamp - $lastActivity > 0) {
            $diffTimestamp = isset($this->data[DeliveryMonitoringService::DIFF_TIMESTAMP]) ? $this->data[DeliveryMonitoringService::DIFF_TIMESTAMP] : 0;
            $diffTimestamp += $lastTimeStamp - $lastActivity;
        }

        $this->addValue(DeliveryMonitoringService::DIFF_TIMESTAMP, $diffTimestamp, true);
    }