Question

Once the user has selected the date from the datepicker, i want to add 41 days.

<script language="javascript">
$(document).ready(function() { 

$("#startdate").datepicker({    dateFormat: 'd/m/y'});

    $('#startdate').datepicker({
        onSelect: function(dateStr) {
            var nights = parseInt($('#numofdays').val());
            var depart = $.datepicker.parseDate('d/m/y', dateStr);
            depart.setDate(depart.getDate('d/m/y') + nights);
            $('#calc').val(depart);
         }
    });




});
</script> 


Start: <input type="text" id="startdate" class="datepicker"><br />
<input type="hidden" id="numofdays" value="41"><br />
Calc: <input type="text" id="calc">

Answer 1

You might just need to combine the options in the same initiating function:

$("#startdate").datepicker({
    dateFormat: 'd/m/y',
    onSelect: function(dateStr, inst) {
        var nights = parseInt($('#numofdays').val());
        var depart = $.datepicker.parseDate('d/m/y', dateStr);
        depart.setDate(depart.getDate('d/m/y') + nights);
        $('#calc').val(depart.toLocaleDateString());
    }
});

See this in action: http://jsfiddle.net/william/L9Szd/.