It will be working fine!
<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="ajax" %>
<asp:ScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ScriptManager>
<asp:TextBox ID="txtStartDate" runat="server"></asp:TextBox>
<ajax:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="txtStartDate"/>
Thanks,
Mamun