I set `ASP` `textbox` text mode to `date`, and noticed its "YEAR" component allows input of up to 9 characters.

My question is, how can limit the year input field to accept a maximum of 4 characters?

```
<asp:TextBox ID="bday" runat="server" 
    TextMode="Date" 
    CssClass="textbox" 
    style="width: 30%; " >
</asp:TextBox>

```