checksum error (press f1)

[johnad]

help please
i have a mesh desktop about 4 years old and at switch on it runs the checks and then says checksum error press F1 to continue and then it all continues fine but the date and time are not set and i have to go and set them every time
any advice please

 

[2scoops0406]

On board CMOS battery is flat perhaps?

 

[eggplant]

probably