How to use timer control in ajax?

 <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:Label ID="Label1" runat="server" ></asp:Label>
        <asp:Timer ID="Timer1" Interval="1000" runat="server" ontick="Timer1_Tick">
        </asp:Timer>

 static int counter=20;
    protected void Timer1_Tick(object sender, EventArgs e)
    {
        counter--;
        Label1.Text = counter.ToString();
}