jQuery/JavaScript function not updating textbox -

i'm using bootstrap datepicker: https://github.com/eternicode/bootstrap-datepicker

i have attached 2 boxes - invdate , invduedate - , pops ok, , sets dates etc fine.

i have drop down box though, when selected, want take date in invdate box, , add val() of dropdownlist invdate, , populate invduedate.

however, cannot seem invduedate change it's date (does nothing) - dropdown box trigger function, line doesn't appear anything:

$("#invduedate").datepicker("setdate", duedate);:

html is:

        <div class="editor-label">         <label for="invdate">invdate</label>     </div>     <div class="editor-field">         <input class="textbox datepicker" data-val="true" data-val-date="the field invdate must date." data-val-required="the invdate field required." id="invdate" name="invdate" type="text" value="02 may 2013" />         <span class="field-validation-valid" data-valmsg-for="invdate" data-valmsg-replace="true"></span>     </div>         <div class="editor-label">        <label for="terms">terms</label>     </div>     <div class="editor-field">       <select id="terms" name="terms"><option value="0">due on invoice date</option>         <option value="7">net 7 days</option>         <option value="14">net 14 days</option>         <option value="21">net 21 days</option>         <option value="28">net 28 days</option>         <option value="45">net 45 days</option>         <option value="60">net 60 days</option>       </select>     </div>      <div class="editor-label">         <label for="invduedate">invduedate</label>     </div>     <div class="editor-field">          <input class="textbox datepicker" data-val="true" data-val-date="the field invduedate must date." data-val-required="the invduedate field required." id="invduedate" name="invduedate" type="text" value="02 may 2013" />          <span class="field-validation-valid" data-valmsg-for="invduedate" data-valmsg-replace="true"></span>     </div> 

my jquery/javascript is:

<script type="text/javascript">   $(document).ready(function () {     $("#terms").change(function () {       var invdate = new date();       var duedate = new date();       invdate = $("#invdate").val();       var net = $("#terms").val();       duedate.setdate(invdate.getdate() + net);       $("#invduedate").datepicker("setdate", duedate);    });   }); </script> 

is obvious i'm going wrong?

update show working code

    var start_date = new date($('#invdate').val());     var duration = parseint($('#terms').val());     var end_date = new date();     end_date.setdate(start_date.getdate() + duration);     $("#invduedate").datepicker("setdate", end_date); 

i think date not correctly formatted, see :

var invdate = new date();  

and it's replaced

$('#invdate').val(); 

but should :

var invdate = new date($('#invdate').val()); 

or (as zeroflagl commented)

var invdate = $('#invdate').datepicker('getdate'); //gives date object