TextBox with CalendarExtender loses value on focus change

G'day guys, I just happened to notice this bug in the system I have built and have been maintaining.... This only happens in the pop-up panels that is used to insert data into the system. The key snippets of the code are included below... <%@ Page Language="VB" AutoEventWireup="false" CodeFile="SalaryClasses.aspx.vb" Inherits="FormData_SalaryClasses" EnableEventValidation="false" StylesheetTheme="Theme" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" > <head id="Head1" runat="server"> ... <body> <form id="form1" runat="server"> <div> ... <asp:Panel ID="pnlSalaryClassPopUp" CssClass="modalPopup" Style="display:none" runat="server" Height="380px&qu ...