The get_wrapperElement() is method of the RadDateInput client-side object which returns the DOM element for the <div> wrapper element. You could access the DateInput of RadDateTimePicker as shown below.
JavaScript:
function enableIt() {
var dtPicker = $find("<%=RadDateTimePicker1.ClientID%>");
var wrapperElement = dtPicker.get_dateInput().get_wrapperElement();