What's with the unintentional time change on my Mac? (laptop, desktop, icon)

[Visvaldis]

This morning when I booted up my Mac G4 I received a prompt that the date/time had changed. According to my Mac it was Dec 31, 1969.
In preferences I put in the correct time and date. I clicked SAVE, and clicked the lock icon to "prevent further changes".
This afternoon when booting up, guess what happened? Yep, it was Dec 31, 1969 again. What gives? Anyone else have this problem?
I can't remember what I did on New Years Eve of 1969, is my Mac trying to tell me something?

[Brian.Pearson]

Good question. Could be a battery, a video card or a bug, judging from the Googling I did. Maybe somebody else will be more helpful.
Link
Link

[Niners fan]

I think that's what happens when the battery that keeps the clock running dies. I have seen that date on my macs before it the battery was the problem. Usually it has happened on my laptop when I let it sleep so long the battery completely ran out.

[Bo]

Originally Posted by Niners fan

I think that's what happens when the battery that keeps the clock running dies. I have seen that date on my macs before it the battery was the problem.

I've seen this many times myself and changed a few batteries to solve it over the years. If it's a desktop Mac, you should be able to find instructions for accessing the battery in the Apple knowledgebase at Apple - Support