copy value from one textbox to another inside UpdatePanel with AJAX CalendarExtender

I have a calendar extender tied to a textbox.  When I select a date from the calendar, it displays the selected date in the textbox.  When I click a submit button, it passes no value.  Any ideas on why the textbox doesn't store the value from the calendar?
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load If Not IsPostBack Then If txtPickUp.Text = "" Then txtPickUp.Text = Date.Today End If If txtDropOff.Text = "" Then txtDropOff.Text = Date.Today End If BuildLocDDL() btnSearch.Attributes.Add("onclick", "return ConfirmL();") End If End Sub
 
<asp:TableCell> <asp:TextBox ID="txtPickUp" runat="ser ...