ajaxToolKit:CalendarExtender not working on row 2 of gridview - asp.net

Question

My page has two approaches to adding a date to a text box inside the gridview. The user can set a date for all the rows by choosing a date and clicking a button. The code behind updates each row. This is working fine. Now I want to add the calendar to each row.
<%# Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="ajaxToolkit" %>
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
This is outside the gridview and working fine.
<asp:TextBox ID="SetDateTextBox" runat="server"></asp:TextBox>
<ajaxToolkit:PopupControlExtender ID="SetDateTextBox_PopupControlExtender" runat="server" BehaviorID="SetDateTextBox_PopupControlExtender"
DynamicServicePath="" ExtenderControlID="" TargetControlID="SetDateTextBox" PopupControlID="Panel1" Position="Bottom">
</ajaxToolkit:PopupControlExtender>
<asp:Panel ID="Panel1" runat="server" Width="200px">
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Calendar ID="Calendar1" runat="server" BackColor="White" BorderColor="Black" BorderStyle="Solid" CellSpacing="1" Font-Names="Verdana" Font-Size="9pt" ForeColor="Black" Height="250px" NextPrevFormat="ShortMonth" Width="330px">
<DayHeaderStyle Font-Bold="True" Font-Size="8pt" ForeColor="#333333" Height="8pt" />
<DayStyle BackColor="#CCCCCC" />
<NextPrevStyle Font-Bold="True" Font-Size="8pt" ForeColor="White" />
<OtherMonthDayStyle ForeColor="#999999" />
<SelectedDayStyle BackColor="#333399" ForeColor="White" />
<TitleStyle BackColor="#333399" BorderStyle="Solid" Font-Bold="True" Font-Size="12pt" ForeColor="White" Height="12pt" />
<TodayDayStyle BackColor="#999999" ForeColor="White" />
</asp:Calendar>
</ContentTemplate>
</asp:UpdatePanel>
</asp:Panel>
Now the problem. I added the calendarExtender to each row inside the templatefield.
<asp:TemplateField HeaderText="FINISH DATE" SortExpression="SCHED_FINISH_DATE">
<EditItemTemplate>
<asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("SCHED_FINISH_DATE", "{0:d}") %>'></asp:TextBox>
</EditItemTemplate>
<ItemTemplate>
<asp:TextBox ID="FinishDateTextBox" runat="server" Text='<%# Bind("SCHED_FINISH_DATE", "{0:d}") %>' ClientIDMode="AutoID"></asp:TextBox>
<ajaxToolkit:CalendarExtender ID="FinishDateTextBox_CalendarExtender" runat="server" BehaviorID="FinishDateTextBox_CalendarExtender"
TargetControlID="FinishDateTextBox"/>
</ItemTemplate>
</asp:TemplateField>
I set the textbox ClientIDMode to AutoID. The page loads without error. I click the textbox on row 1 and the calendar works great. Click row 2 and beyond and the calendar does not appear.

Answer 1

I removed the popupcontrolextender along with the asp:panel and asp:UpdatePanel and replaced with the following code. This calendar will set a global change to all rows. So much cleaner.
<asp:TextBox ID="SetDateTextBox" runat="server"></asp:TextBox>
<ajaxtoolkit:calendarextender ID="CalendarEntender2" runat="server" TargetControlID="SetDateTextBox" />
Then inside the gridview.
<asp:TemplateField HeaderText="FINISH DATE" SortExpression="SCHED_FINISH_DATE" ItemStyle-Width="100">
<EditItemTemplate>
<asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("SCHED_FINISH_DATE", "{0:d}") %>'></asp:TextBox>
</EditItemTemplate>
<ItemTemplate>
<asp:TextBox ID="FinishDateTextBox" ClientIDMode="AutoID" runat="server" Text='<%# Bind("SCHED_FINISH_DATE", "{0:d}") %>'></asp:TextBox>
<ajaxtoolkit:calendarextender ID="CalendarEntender1" runat="server" TargetControlID="FinishDateTextBox" />
</ItemTemplate>
<ItemStyle Width="100px"></ItemStyle>
</asp:TemplateField>
I still don't understand why the popupextender was causing an issue.