I think one of the other suggestions will correct the problem. However, do check date and time Daylight savings and all.
Skydancer mentioned battries. How old is your machine? Have you ever changed the battery? They do lose power at some point and this changes the date/time. Usually an easy change (or a good excuse to upgrade to a newer computer!)
Good luck
My question is: why does my drive activity light just keeps running and running ?