was the clock/calender working before you fitted the new battery?
If so, I would suspect the battery is deficient, or not fitted correctly.
Stuart's suggestion regarding the Y2k problem is reasonable. Try proving it, by setting a date in 1995, or similar, and see what happens when the pc is next switched on.
If you are only running dos on it, without internet access, I would not expect it to change the date to current.
As a matter of interest, where are you located? I am in Hammersmith, West London.
Good luck - Keep us posted.