Calculate end date from start date and radio button values

Using FormCalc, I wish to calculate from a user entered start date, adding 90, 180 or 365 days to get to get an end date. I am using radio buttons for the 90, 180 and 365 days.  So far I have this:

(radioButton.rawValue==1)//radio button bound to 1 is on/yes then;//

and I have this:

current=Date2Num(Start_Date.rawValue,"MM/DD/YYYY")

future=current+90

newDate=Num2Date(future,"MM/DD/YYYY")

= newDate

but I don't know how to put them together or incorporate the 180 and 365 days.  I know the second part works because I currently use it, but that is as far as I have gotten.

Any help/suggestions will be greatly appreciated!!!

Thanks,

Lou Johnson