js倒计时

function timer()  
{  
    var ts = (new Date(2018, 11, 11, 9, 0, 0)) - (new Date());//计算剩余的毫秒数  
    var dd = parseInt(ts / 1000 / 60 / 60 / 24, 10);//计算剩余的天数  
    var hh = parseInt(ts / 1000 / 60 / 60 % 24, 10);//计算剩余的小时数  
    var mm = parseInt(ts / 1000 / 60 % 60, 10);//计算剩余的分钟数  
    var ss = parseInt(ts / 1000 % 60, 10);//计算剩余的秒数  
    dd = checkTime(dd);  
    hh = checkTime(hh);  
    mm = checkTime(mm);  
    ss = checkTime(ss);  
    document.getElementById("leftTime").innerHTML = dd + "天" + hh + "时" + mm + "分" + ss + "秒";  
    setInterval("timer()",1000);  
}  
function checkTime(i)    
{    
   if (i < 10) {    
       i = "0" + i;    
    }    
   return i;    
}