Add days to date using Javascript

I am trying to add days to a given date using Javascript. I have the following code:

function onChange(e) {
    var datepicker = $("#DatePicker").val();
    alert(datepicker);
    var joindate = new Date(datepicker);
    alert(joindate);
    var numberOfDaysToAdd = 1;
    joindate.setDate(joindate + numberOfDaysToAdd);
    var dd = joindate.getDate();
    var mm = joindate.getMonth() + 1;
    var y = joindate.getFullYear();
    var joinFormattedDate = dd + '/' + mm + '/' + y;
    $('.new').val(joinFormattedDate);
}

On first alert I get the date 24/06/2011 but on second alert I get Thu Dec 06 2012 00:00:00 GMT+0500 (Pakistan Standard Time) which is wrong I want it to remain 24/06/2011 so that I can add days to it. In my code I want my final output to be 25/06/2011.

Fiddle is @ http://jsfiddle.net/tassadaque/rEe4v/