There are two things that could cause the clock to gain time.
1) The Newton was previously in a diagnostic mode. The clock's speed is too fast in the diagnostic mode for everyday use. If the Newton is not reset after leaving diagnostic mode, then the clock continues to run fast. Reseting the Newton returns the clock to normal speed.
2) A faulty battery cell also causes the clock to skip ahead. Usually this happens if you are using the rechargeable battery pack but it can also happen with alkaline batteries. If you are using a rechargeable battery pack, try to drain the battery and then recharge the battery in the Newton recharger.
If the clock still has problems, have the rechargeable battery pack serviced. One of the battery cells may not be charging properly. If you are using alkaline batteries, make sure all the batteries are from the same package. You should not mix old and new batteries.