html

<div>
    <span id="show_info">正在发送升级命令，请勿刷新页面！</span>
    <span id="timedown"></span>
</div>

javascript

function checkTimeDown(time)
{
    var timeId;
    if(time==="undefined"){
        time=60;
    }
    $("#timedown").html(time+"秒");
    time = time-1;
    if(time>=0)
    {
      if(timeId){window.clearTimeout(timeId);}
      timeId = setTimeout("checkTimeDown("+time+")",1000);
    }else{
       window.clearTimeout(timeId);
    }
}

倒计时

原文：http://my.oschina.net/wbo0801/blog/336644