date related error

I am trying to take a field, that is a date, that represents the date of the last order from a particular customer and to that add a calculated number of days, a number, expected til receipt of the next order.

So the formula is {@Last Total Ord Date}+({@New Total Days}). This occasionally gives me an error that states "the number of days is out of range".

An instance in which it does NOT give the error is with a customer whose last order date was 10/31/11 and whose number of days til next order is 121. It correctly gives me the result of 02/29/12.

The error is resulting with the next customer, in an instance where that customer's last order date was 11/16/11 and whose number of days til next order is 60.

Awfully confused here......sorry if I appear to be the newbie that I am.

Help would be greatly appreciated. Thank you in advance!

Ted