How do I fix this simple calendar example?

   Why does the following code give this message?
String was not recognized as a valid DateTime.
 
 <div>
            <asp:Button ID="btnDate" runat="server" Text="Button" />
            <asp:TextBox ID="EventDateTextBox" runat="server">
            </asp:TextBox>
            <asp:Calendar ID="clnEvent" runat="server" Visible= "False">
            </asp:Calendar>
    </div>
 
VB Code
 
Partial Class clnControl
 & ...