定时显示按钮的代码

    <form action="#" method="post" name="agree">
<input type="submit" value="请认真查看<服务条款和声明> (15 秒后继续)" name="agreeb">　　
    <input onclick=history.back(-1) type="reset" value=" 我 不 同 意 "></form>
<SCRIPT language=javascript>
<!--
var secs = 15;
document.agree.agreeb.disabled=true;
for(i=1;i<=secs;i++) {
window.setTimeout("update(" + i + ")", i * 1000);
}
function update(num) {
if(num == secs) {
document.agree.agreeb.value =" 我 同 意 ";
document.agree.agreeb.disabled=false;
}
else {
printnr = secs-num;
document.agree.agreeb.value = "请认真查看<服务条款和声明> (" + printnr +" 秒后继续)";
}
}
//-->
</SCRIPT>