ASP.NET AJAX Change a textbox two times through one lifecycle

Hello,

I would like my button to change a textbox two times during one lifecycle, Meaning,
When I click it, I would like first to see in the textbox - "start" and then after a few seconds "end"
However, with my following I only get "end" after the loop ends (take few seconds)

I understand that's how ajax lifecycle works, is there any way someone could suggest to manipulate it?

This is my code:
    <asp:UpdatePanel ID="upResult" runat="server">
        <ContentTemplate>
                        <asp:TextBox runat="server" ID="Date" Width="50px"/>
                       <asp:Button ID="Button" runat="server" Text="Button" onclick="Button_Click" />
      </ContentTemplate>
    </asp:UpdatePanel>

    protected void Button_Click(object sender, EventArgs e)
    {
        Date.Text = "start";        

        int i=0;
        while (i < 1000000000)
        {
            i++;            
        }

        Date.Text = "end";      
    }


Thanks a lot!