Advanced Formula to calculate a date

We are on Unifier 20.12.10. We have 2 data elements that we currently use to calculate a 3rd element. Element 1 is a First Chargeable Day (Date Only definition), Element 2 is a Proposed Time (Decimal Amount definition), and Element 3 is the Original Completion Date (Date Only definition).

Currently, the Formula function for a date element adds the date, plus the number of days, and gives us the 3rd date, but unfortunately, it's calculating incorrectly for our use. For example, if we say our first chargeable day is Jan 1, and the proposed time is 10 days, Unifier is providing us with a completion date of Jan 11. However, that isn't correct. It seems to be ignoring either the first or last day in the calculation. The First Chargeable Day should be counted as day one, so the 10th day would be January 10.