Jquery Ui Date Picker

Hi,

I need a small help i have two date pickers start date and end date by this i calculate week and show in a textbox.

The problem is if i change week for e,g 5 to 4 the end date should also change similarly if i add i make it 5 to 6 it should increase.

A Fiddle to go with it: http://fiddle.jshell.net/WU9P6/24/