Hi Gert, I think you meant to say you wanted to calculate the new Expiry date as the Start Date would be the starting point for the lease

This is the formula you would put in the Expiry Field. I'm assuming your Variable names matches the field names.

=DATE(YEAR(Start_Date)+Lease_Period,MONTH(Start_Date),DAY(Start_Date))

If you have no default value in the Lease Period Field this would produce the dreaded Calculation Field error, so to get around that you may add an If statement such as this

=IF(Period="",Start_Date,DATE(YEAR(Start_Date)+Lease_Period,MONTH(Start_Date),DAY(Start_Date)))

If you used a single line field instead of the Date field in Forms this opens us some different options as you must put a date value in a date field which is why I just populated it with the Start Date in the case that the Lease Period field was empty (no value selected)

I'd also suggest you set the Expiry field as Read only so that it can;t be changed manually by the user.

Hope this helps