If it happens that the Date and time on the Gigaset T300/T500 is slowly changing to a wrong value, you should check if below procedure solves the problem. if this is not the Case, you need to create an RMA request.
Make a SSH connection with the PBX.
Now set the time manually via Admin-Webinterface
Reset the Hardware-Clock:
Not set the time to NTP via Admin-Webinterface again