Dove into FormCalc and answered my own question.
var pDate = Date2Num(Date A.formattedValue,"MM/DD/YYYY")
var mDate = Date2Num(Date B.formattedValue,"MM/DD/YYYY")
var kDays = mDate - pDate
if (kDays >= 90) then Num2Date(mDate - 15)
elseif ((kDays >= 30) & (kDays<= 89)) then Num2Date(mDate - 10)
else Num2Date(mDate - 5) endif
I'm managing the pop-up in a secondary way.