Oh, sorry. I overlooked you last sentence.
To accomplish this, you'll need antother script. However, FormCalc makes data calculations a breeze.
Put this in the exit event of the "To" field.
if (not $.isNull) then
var n = Date2Num($.formattedValue, "MM/DD/YY")
var d = Num2Date(n, "D")
var m = Num...