<head>
<SCRIPT LANGUAGE="JavaScript">
var maxtime = 30*30 //半個小時，按秒計算，自己調整!
function CountDown(){
if(maxtime>=0){
minutes = Math.floor(maxtime/30);
seconds = Math.floor(maxtime%30);
msg = "距離結束還有"+minutes+"分"+seconds+"秒";
document.all["timer"].innerHTML=msg;
if(maxtime == 5*30) alert('註意，還有5分鐘!');
--maxtime;
}
else{
clearInterval(timer);
alert("時間到，結束!");
}
}
timer = setInterval("CountDown()",1000);
</SCRIPT>
</head>
<body>
<div id="timer" style="color:red"></div>
</body>
</html>