Calendar onSelectionChanged

I  want to use a calendar control for entering date
<asp:TextBox ID="OdTextBox" runat="server" Text="" ></asp:TextBox>
<asp:Button  ID="ButtonOd"  OnClick="Od_onClick" runat="server"  Text="..."    />
<div  style="position:absolute">
<asp:Calendar ID="CalendarOd" visible="false" OnSelectionChanged="CalendarOd_SelectionChanged"  runat="server" >
</asp:Calendar>
</div>

code behind:

   Sub Od_onClick(ByVal sender As Object, ByVal e As EventArgs)
        CalendarOd.Visible = True
    End Sub

    Sub CalendarOd_SelectionChanged(ByVal sender As Object, ByVal e As System.EventArgs)
       OdTextBox.Text = ...