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.

Delete NTP-Offset-File:

[root@tel ~]# rm /etc/adjtime

Now set the time manually via Admin-Webinterface

Reset the Hardware-Clock:

[root@tel ~]# hwclock --systohc

Not set the time to NTP via Admin-Webinterface again