I actually found out the answer myself, 18 months = 548 days, 13 months = 396 days, 15 months = 457 days. I tried it many times and the numbers are correct so I simpled it down to the following code: var date = Date2Num(StartDate.formattedValue,"DD/MM/YYYY")EndDate.formattedValue = Num2Date(date+548...