asp.net页面如何实现按钮倒计时

具体页面代码如下:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>按钮倒计时</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Button ID="Button1" runat="server" Text="Button" /></div>
        <script type="text/javascript" language="javascript">
            var secs = 10;
            var wait = secs * 1000;
            var agreetext="同意";
            document.form1.Button1.value = agreetext+"(" + secs + ") ";
            document.form1.Button1.disabled = true;
            for(i = 1; i <= secs; i++) {
             window.setTimeout("update(" + i + ")", i * 1000);
            }
            window.setTimeout("timer()", wait);
            function update(num, value) {
             if(num == (wait/1000)) {
             document.form1.Button1.value = agreetext;
             } else {
             printnr = (wait / 1000)-num;
             document.form1.Button1.value = agreetext+"(" + printnr + ") ";
             }
            }
            function timer() {
             document.form1.Button1.disabled = false;
             document.form1.Button1.value = agreetext;
            }
      </script>
    </form>
</body>
</html>

注意以下两点:
1、setTimeout(表达式,延时时间)
在执行时,是在载入后延迟指定时间后,去执行一次表达式。
2、javascript必须放在<form>与</form>内。
3、在<form>与</form>内的JAVA脚本才能使用“document.form1.服务控件名”来引用页面中的服务类型的控件。如果将JAVA脚本放在<head>节内是不能使用这种形式来引用控件的。