ajax calendar extender saving only default date into DB

aspx

 <tr>
            <td>
                DiscoveryDate
            </td>
            <td class="style1">
                <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">  
                </asp:ToolkitScriptManager>
                <asp:TextBox ID="txtcalDiscoveryDate" runat="server"></asp:TextBox>
                <asp:Image ID="Image2" runat="server" ImageUrl="~/images/cal.png" />
                <asp:CalendarExtender ID="CalendarExtender1" TargetControlID="txtcalDiscoveryDate" Format="dd/MM/yyyy" PopupButtonID="TextBox1" PopupPosition="Right" runat="server" />
            </td>
        </tr>

cs:

protected void bnRegisterApplication_Click(object sender, EventArgs e)  {

a.DiscoveryDate = CalendarExtender1.SelectedDate.GetValueOrDefault(); }

its getting saved as 1/1/0001 12:00:00 AM in the DB even after selecting the date. I see in debugging its getting the date into a.DiscoveryDate but CalendarExtender1.SelectedDate.GetValueOrDefault() returning deafult. Shouldnt I be using the extender there?

I cant use  a.DiscoveryDate = txtcalDiscoveryDate.Text; bz the error "cant implicity convert type "string" to System.Date.Time"

Any direction where I can check.