A mobo battery's life is measured in years rather than months - I've never had to replace one over the past eight years and three different mobos.
It may be that Windows' automatic date and timekeeping mode (once correctly set) has gone haywire somewhere (I'm assuming that XP is the same as 98 etc).
Normally when the clocks go back or forward Windows updates automatically and a panel comes up when you next switch on for you to confirm the new time.
Why not download one of the atomic clock programs (very small) and use that instead? It will be updated every time you switch on, just like the clocks which check with the Rugby time signal.