Unless you want to combine all three into a common design then no.
Date arithmethic is quite simple using FormCalc. You need to use the Date2Num function. This will give you t he number of days that have elapsed since a set point in time (called the epoch). When you have that you can add the number of days that you want then use num2Date to turn that number back into a date. The code woudl look something like this:
// Get the number of days from the epoch for the starting date
var startDays = Date2Num(DateField.rawValue, "YYYY-MM-DD")
// add 14 to the number of days
var finishDays = startDays + 14
//Change the result back into a date
FinishDate.rawValue = Num2Date(finishDays, "YYYY-MM-DD")
If you want to use Javascript it can be done but it is much more involved.