How to get date picker value, add day number from text field and display the result on another text field or datepicker?

Hi,

I need to add warranty date on a list. So the operator will fill the purchase date (datepicker) and warranty length (text), and display it on the same form on another text field or datepicker field. I try the solution from /questions/22085936/answers/22085944 but did not work as expected. Looks like the event   onSelect: function( selectedDate ) not triggered. Any idea fo how to implement this?