Ok, lets try to do this blind - don’t worry, the BIOS is pretty well locked down, so unlikely to do any harm if it goes wrong.
Switch the unit on with your monitor connected & press the delete key above the direction arrows when you reach the CMOS error screen you posted above.
Give it 10 seconds or so.
Using the direction arrow keys:
Press right once.
Press down three times.
Press Return.
Press the Y key.
Press return.
Direction down arrow once.
Press return twice.
Hopefully the unit should then boot up.
Then shutdown & restart.
If the unit then boots without issue, then use the Naim menu system to set the date & time.
Best of luck!
Regards
Neil.