Why does AJAX CalendarExtender display behind other controls?

<!--In this example, CalendarExtender2 is tied to the asp.net textbox control "txtDate".  When the calendar drops down, it displays behind other textboxes and dropdown list boxes.  It displays over text.-->
 
<%@ Page Language="VB" MasterPageFile="~/COS.master" AutoEventWireup="false" CodeFile="StatTab.aspx.vb" Inherits="StatTab" title="Untitled Page" %>
 
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
	Namespace="System.Web.UI" TagPrefix="asp" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
	&nbsp;
	<asp:ScriptManager ID="ScriptManager1" runat="server">
	</asp:ScriptManager>
	<cc1:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="1" Style="position: relative" Width="90%">
		<cc1:TabPanel ID="TabPanel1" runat="server" HeaderText="TabPanel1">
			<ContentTemplate>
				<asp:UpdatePanel ID="UpdatePanel2" runat="server">
					<ContentTemplate>
						HEADER INFO:<br /><br />
						
						<table style="width: 100%; position: relative">
							<tr>
								<td colspan="2">
									Current
									Date:</td>
								<td colspan="4">
									<asp:TextBox ID="txtDate" runat="server" Style="position: relative"></asp:TextBox>
                                    <cc1:CalendarExtender ID="CalendarExtender2" runat="server" TargetControlID="txtDate">
                                    </cc1:CalendarExtender>
                                </td>
							</tr>
							<tr>
								<td style="width: 100px">
									Last Name:</td>
								<td colspan="2">
									<asp:TextBox ID="txtLast" runat="server" Style="position: relative"></asp:TextBox></td>
								<td style="width: 100px">
									First Name:</td>
								<td colspan="2">
									<asp:TextBox ID="txtFirst" runat="server" Style="position: relative"></asp:TextBox></td>
							</tr>
							<tr>
								<td style="width: 100px">
									Job Title:</td>
								<td colspan="2">
									<asp:TextBox ID="txtTitle" runat="server" Style="position: relative"></asp:TextBox></td>
								<td style="width: 100px">
									SSN:</td>
								<td colspan="2">
									<asp:TextBox ID="txtSSN" runat="server" Style="position: relative"></asp:TextBox></td>
							</tr>
							<tr>
								<td style="width: 100px">
									Office Location:</td>
								<td colspan="2">
									<asp:DropDownList ID="ddlLocation" runat="server" Style="position: relative" AutoPostBack="True">
										<asp:ListItem Value="JAK">Jackson</asp:ListItem>
										<asp:ListItem Value="MAD">Madison Heights</asp:ListItem>
										<asp:ListItem Value="RVR">RiverCrest</asp:ListItem>
										<asp:ListItem Value="CHI">Clubhouse Inspiriation</asp:ListItem>
										<asp:ListItem Value="OXF">Oxford Main</asp:ListItem>
										<asp:ListItem Value="FNT">Flint</asp:ListItem>
										<asp:ListItem Value="SAG">Saginaw</asp:ListItem>
										<asp:ListItem Value="BSL">Bayside Lodge</asp:ListItem>
										<asp:ListItem Value="OTH">Other</asp:ListItem>
									</asp:DropDownList></td>
								<td style="width: 100px">
									If Other...</td>
								<td colspan="2">
									<asp:TextBox ID="txtLocationOther" runat="server" Style="position: relative"></asp:TextBox></td>
							</tr>
						</table>
					</ContentTemplate>
				</asp:UpdatePanel>
			</ContentTemplate>
			<HeaderTemplate>
				Header
			</HeaderTemplate>
		</cc1:TabPanel>
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</asp:Content>