Javascript Add variable # Days to Calendar then populate field

I have a Great handling idea for a user experience that needs to calculate EXACTLY 30 days from any given date, but though I know Im close, I cannot figure out whats NOT working...

If I populate one form input field with todays date so a user can change it if need-be,
then when they go to click the dropdown to extend that date. The script should grab the original date "origdate" and onChange of the drop down go perform the calculation of the selected number of days and add that to the original date then populate the changed extended date "edate" before pressing UPDATE btn to continue.


thanks so much


P.S. please do not introduce/suggest a bulky javascript framework -- not my idea of working it out w/ DOM. thanks so much!!

<script type="text/javascript">
function setRetention(duration) { 
	var lengthRetention = ""; 
	var newAssignment = ""; 
	var tempOrigDate = ""; 
  if (duration.selectedIndex>0) { // e.g. not the "Select Option"
     //works
     lengthRetention += duration.options[duration.selectedIndex].value;
     
     // not too sure if functioning right...
     tempOrigDate += document.forms[0].origdate.value;
     // or tempOrigDate += document.getElementsById('cdate').value;
     // think its failing cause maybe Im not handling data-types correctly      
     tempOrigDate.setDate(tempOrigDate.getDate()+lengthRetention);
     // adding this way makes sense right? integer 30 days + to date right?
     
  }
   // then I want to push that new calculated value to a display form field
  duration.form.edate.value=newAssignment;
}
</script>



<%
' ASP.Net code 
today = month(now) & "/" & day(now) & "/" & year(now)
Response.Write "<form name='edit' action='writetodb.asp' method='post'>"
'
Response.Write "Original Date: <input name=""origdate"" id=""origdate"" type=""text"" value='" & today & "'>"
'
Response.Write "Assign an extended Date:<br>"
Response.Write "<select name=""retention"" id=""retention"" onChange=""setRetention(this)"">"
Response.Write "<option selected=""selected"">Select an Extension Length</option>"
Response.Write "<option value=""30"";>30 days Active</option>"
Response.Write "<option value=""14"";>14 days Active</option>"
Response.Write "</select><br>"
'
Response.Write "Expiration Date: <br>"
'
' The Revised Date should populate and be displayed in this input field::.
Response.Write "<input type=""text"" name=""edate"" id=""edate"">"
'
Response.Write "<input type=submit value='UPDATE'>"
'
%>
</form>

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