ToolTip For RadDatePicker

Hi, 
I currently experiencing one problem for change the toolTip property of RadDatePicker control 

code as follows:
<telerik:GridTemplateColumn HeaderText="<%$ Resources:WebUIResources, I18N_Date %>" UniqueName="DateOfAttendance" DataField="DateOfAttendance" SortExpression="DateOfAttendance" Groupable="False">
                <ItemTemplate>
                    <%# String.Format("{0:ddd}", Eval("DateOfAttendance")) + " - " + String.Format("{0:d}", Eval("DateOfAttendance")) %>
                </ItemTemplate>
                <EditItemTemplate> 
                    <telerik:RadDatePicker ID="DateOfAttendance" runat="server" DbSelectedDate ='<%# Bind("DateOfAttendance") %>'  SkipMinMaxDateValidationOnServer="True"  MinDate="1800-01-01" MaxDate="9999-12-31" Width="100%" />
                    <telerik:RadToolTip ID="DateOfAttendance_RadToolTip" runat="server" IsClientID="true"     TargetControlID="DateOfAttendance_wrapper" Text="show tooltip" ShowEvent="OnMouseOver">
                   </telerik:RadToolTip> 
                </EditItemTemplate>
            </telerik:GridTemplateColumn>  But mouse over on the RadDatePicker, the display content still the default one, seems RadToolTip is not working. anyone can Help? all the controls inside teleGrid Control