Calculating dates and number of working days

Dear Expert,
 
I have created a form using Live Cycle Designer 8.0. I am trying to let it do a simple calculation but because I am not keen in Javascript, I don't seem to get it right. All I wanted to do is for users to input Start date of a task as well as inputting the number of working days on the task, and I just wanted a script to automatically plot the completion date of that task.
Alternatevly, I don't mind that users to input both dates (start and completion) and the script would calculate the number of working days for that task...
 
Please help as I already have corrupted my pdf file by messing about with lots of different scripts and thank God I have made a back up copy before attempting my failed script making!!!
 
Thanks in advanced...