I am sorry you are having problems with this unit but let me help you.
Based on your description I would say that the problem is the motherboard battery. When the batter fails the system starts behaving strange and just like the issue you are experimenting. I would recommend changing the battery for a new one.
Another thing that can be affecting is that Windows have an option under Date and Time that synchronizes the Time and date with an internet Time server so disabling this option may help.
Thanks for response but as already mentioned, I tried replacing battery & have also kept machine permanently connected to the mains but even after having cleared down the BIOS error log, still get the errors & corrupt date/time.
What I didn't mention was that I have also booted computer to DOS and running the DATE command still shows corrupt year so not a Windows or time server problem.
None of this would explain why the date/time fields are missing from the BIOS either.
Any other thoughts?