asp:TextBox and ajaxToolkit:CalendarExtender, so the page should be look like:
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<ajaxToolkit:ToolkitScriptManager ID="ScriptManager1"runat="server" />
<div>
<asp:TextBox ID="Date1" runat="server"></asp:TextBox>
<ajaxToolkit:CalendarExtender
ID="CalendarExtender1" runat="server"TargetControlID="Date1">
</ajaxToolkit:CalendarExtender>
</div>
</form>
</body>
</html>
Note that TergetControlID point to the text box id.
