How can I get Word 2007 to give a correct ending date?

I'm drafting a lease template using Word 2007 form fields. User will input a start date and how many years the lease will run. Now, I'm needing help in writing the IF statement for the ending date. It needs to follow along the lines of: IF the start date is on the first of the month, then the ending date will be on the first day of the month however many years the lease run is from start, but if the start date is on any other date not the first of the month, then the ending date will be on the last day of the month that the start date was in but however many years the lease run is from start.

So i.e. if the start date was 1/1/2020 and the lease run is 5 years, then the end date should be 1/1/2025. If the start date was 1/2/2020 with the same 5 year lease run, then the end date should be 1/30/2025. Obviously, the formula will have to read actual calendar dates to account for months having 30 or 31 days and Feb. with its 28 or 29 days.