Get value of ASP.NET hidden field using jQuery.

jQuery_ASP_HiddenField.js

function DateSelected(sender, eventArgs) {
  // get selected date (this is a Telerik RadDatePicker object) 
  var selectedDate = eventArgs.get_newDate();

  // get value of the years until expiration of a valuation (asp hidden field control)
  var expirationYears = $('#<%=hfExpirationYears.ClientID%>').val();
            
  //derive months from years
  var months = expirationYears * 12;

  // calculate a date that is x months from the selected date
  var expirationDate = new Date(new Date(selectedDate).setMonth(selectedDate.getMonth() + months));

  //format the date display
  var labelDisplay = expirationDate.toString("M/d/yyyy");

  // update the expiration date on the page (updates an ASP.NET label control)
  $("#<%=lblExpirationDate.ClientID%>").html(labelDisplay);
}