how to add days / weeks from separate field in start date to get end date in datepicker?

Question

In my form , I have 3 fields.

<input name="course_duration" id="course_duration" type="text"> (A numeric value which denotes 'Weeks'
<input name="course_start" id="course_start" type="text">
<input name="course_end" id="course_end" type="text">

I am using datepicker jquery and I want to fillup course_end date automatically by adding number of weeks from value inserted in course_duration field + course_start date

currently I am using following javascript code to popup calendar and selecting dates for course_start and course_end manually.

<script type="text/javascript">
$(document).ready(function() {
$('#course_start').datepicker({dateFormat: 'yy-mm-dd'});
$('#course_end').datepicker({dateFormat: 'yy-mm-dd'});
});
</script> 

JSFIDDLE

Answer 1

// Get the week from course_duration
var weeks = $('#course_duration').val();

// Get the selected date from startDate
var startDate = $('#course_start').datepicker('getDate');
var d = new Date(startDate);

// Add weeks to the selected date, multiply with 7 to get days
var newDate = new Date(d.getFullYear(), d.getMonth(), d.getDate() + weeks * 7);

// Set the new date to the course endDate
$('#course_end').datepicker('setDate', newDate);

Demo