I have created the form like below
Star Date : DatePicker
ADd days: Textbox
So when the user selects a start date and Add days then the Due date should be displayed based on the calculation of Startdate+add days.
For ex: Is start date is 6/23/2013 and Add days is 7 then due date should be 6/30/2013.
Can you give formule in infopath form or a list.