<script type="text/javascript" src="http://www.cnblogs.com/Content/Phone/js/Answer.js?201402242021"></script>

<script type="text/javascript">

    $(function () {

        timer;

    })

    var maxtime = 20 * 100;

    function CountDown() {

        if (maxtime >= 0) {

            seconds = Math.floor(maxtime / 100);

            milliseconds = Math.floor(maxtime % 100);

            seconds = seconds < 10 ? ("0" + seconds) : seconds;

            milliseconds = milliseconds < 10 ? ("0" + milliseconds) : milliseconds;

            $(".time").text(seconds + ":" + milliseconds);

            //if(maxtime == 5*60) alert('注意，还有5分钟!');

            if (maxtime == 270) {

                //$("#musicNear")[0].play();

            }

            maxtime -= 10;

        }

    }

    timer = window.setInterval(CountDown, 100);

</script>