Originally posted by Paul the Sunman "The date and time may be reset if you leave the battery out of the camera for long periods of time. If this happens, set the current date and time again."
There will be a small internal battery that is sufficient to keep time when the main battery is removed, but that will eventually exhaust. Perhaps yours is weakened/damaged.
That was my first thought, too. I have had this issue with a couple of compacts. One is reset every time I remove the battery even for a brief time, and I think one I had years ago prompted for date and time each time it was switched on even without having the battery removed.