JS 考试倒计时

if(confirm("开始考试！")!=true)
{
    window.location.href = "";
}


var maxtime = 90*60 //一个小时，按秒计算，自己调整!
function CountDown(){
if(maxtime>=0){
minutes = Math.floor(maxtime/60);
seconds = Math.floor(maxtime%60);
minutes = minutes>=10?minutes:'0'+minutes;
seconds = seconds>=10?seconds:'0'+seconds;
msg = "考试剩余时间："+minutes+"分"+seconds+"秒";
document.all["navbar"].innerHTML=msg;
if(maxtime == 5*60) alert('注意，还有5分钟考试结束!');
--maxtime;
}
else{
clearInterval(timer);
alert("时间到，考试结束，请交卷!");
return false;
}
}