Exception: 'AjaxControlToolkit.CalendarExtender' cannot extend controls of type 'System.Web.UI.WebControls.DropDownList'
that is because the calendar extendar is designed to work with a textbox. If you really want the result in a drop down list you could try putting a textbox on your field and making it hidden. Then attach the calendar extender to that hidden textbox and when the user clicks a date, assign the drop down list to the value of the textbox.
