After adding javascript,kindly add the below code in form tags.

3.Copy the below code in between the form tag

<form runat="server">
  <div>
  
     <asp:ScriptManager ID="ScriptManager1" runat="server">
            </asp:ScriptManager>
              <asp:UpdatePanel ID="updOptions" runat="server">
                        <Triggers>
                            <asp:PostBackTrigger ControlID="btnSubmit" />
                        </Triggers>
                        <ContentTemplate>
     <asp:TextBox runat="server" ID="txtExpDt"  TabIndex=4 Width="98px" MaxLength="10"></asp:TextBox>
     <cc1:CalendarExtender ID="CalendarExtender1" OnClientDateSelectionChanged="checkDate" TargetControlID="txtExpDt" Format="MM/dd/yyyy" runat="server">
     </cc1:CalendarExtender>
     <cc1:MaskedEditExtender ID="MaskedEditExtender1" runat="server" TargetControlID="txtExpDt"
     Mask="99/99/9999" MessageValidatorTip="true" OnFocusCssClass="MaskedEditFocus"
     OnInvalidCssClass="MaskedEditError" MaskType="Date" ErrorTooltipEnabled="True" />
        
     <cc1:MaskedEditValidator ID="MaskedEditValidator1" runat="server" ControlExtender="MaskedEditExtender1"
     ControlToValidate="txtExpDt" IsValidEmpty="true" InvalidValueMessage="Expiry should be greater than today"
     Display="Dynamic" EmptyValueBlurredText="&nbsp;" InvalidValueBlurredMessage="&nbsp;" Visible="False"
     />
       <asp:Button ID="btnSubmit" runat="server" TabIndex=26 Width="100px" Text="Submit"
                                                         onclick="btnSubmit_Click" 
                                                         />
<asp:Label id="lblMessage" runat="server" text=""></asp:Label>
     </ContentTemplate>
     </asp:UpdatePanel> 
   </div>
   </form>