<%--倒计时--%>

<script type="text/javascript" language="JavaScript">
    secs = 59; // Number of secs to delay -CHINA-studio    
    wait = secs * 100;
    minute = 3; //考试总时间

    //循环调用秒的减方法
    cycle();

    function cycle() {
        for (i = 0; i <= (wait / 100); i++) {
            //将i*1000改为i*100查看最后的效果
            window.setTimeout("doUpdate(" + i + ")", i * 1000);
        }
    }

    function doUpdate(num) {
        if (num == (wait / 100)) {
            cycle();
            //三分钟前提醒考生交卷
            if (minute == 3) {
                alert("还有三分钟时间，请抓紧时间答卷！");
            }
            //时间到，提交表单
            if (minute <= 0) {
                //   document.exam.action="";
                document.all.btnhandin.click();
            }
            minute--;
        } else {
            wut = (wait / 100) - num;
            //使得输出格式为00:00
            s = wut < 10 ? "0" + wut : wut;
            m = minute < 10 ? "0" + minute : minute;
            //将剩余时间显示在页面上
            document.getElementById('lefttime').innerHTML = "剩余时间： + m + "分" + s + "秒";
        }
    }
  
</script>

<td>
<asp:Label ID="lefttime" runat="server" Text="Label"></asp:Label></td>
                                    </tr>