js 中的倒计时功能

var maxtime = 45 * 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);
                    msg = "剩余时间：<span style='font-size:25px; color:red;font-family: Arial;'><strong>" + minutes + ":" + seconds + "</strong></span>";
                    document.all["timer"].innerHTML = msg;
                    if (maxtime == 5 * 60) alert('注意，还有5分钟!');
                    --maxtime;
                }
                else {
                    clearInterval(timer);
                    alert("考试时间到!");
                    submit_Exam();
                }
            }
            timer = setInterval("CountDown()", 1000);  

<div id="timer" style=" font-size:13px;"></div>