Anyone know why my Mac's date/time reverts back to December 31st, 1969 everytime it dies from a low battery?

Weird right?

I have a Mac Powerbook G4 and every time it dies from a low battery, it reverts back to December 31st, 1969 at 7:00PM. I’m sure it was a great year, and time, but I like it here in 2009! Anyone know why this happens? I mean, computers weren’t even around then, so I know it’s not the ghost of computer past…