Hi I got an answer from the support of Dlink and they said that the camera has no internal battery hence the time loss after unplugging from power supply.
If I synchronization is activated to an NTP-Server, it seems to take several hours until synchronization is done. This does not suit our needs as we want to power on the camera and have recorded it within a minute after it.
Is it the fault of the firmware that the synchronization takes many hours or is it the NTP-Server? If so, which server provides "real time" synchronization for Central European Time?