JS的倒计时和已过时

<script language="javascript" type="text/javascript"> 
var interval = 1000; 
function ShowCountDown(year,month,day,divname) { 
var now = new Date(); 
var endDate = new Date(year, month-1, day); 
var leftTime= now.getTime()-endDate.getTime();//倒计时反减
var leftsecond = parseInt(leftTime/1000); 
var day1=Math.floor(leftsecond/(60*60*24)); 
var hour=Math.floor((leftsecond-day1*24*60*60)/3600); 
var minute=Math.floor((leftsecond-day1*24*60*60-hour*3600)/60); 
var second=Math.floor(leftsecond-day1*24*60*60-hour*3600-minute*60); 
var cc = document.getElementById(divname); 
cc.innerHTML = "离2018年1月1日已过："+day1+"天"+hour+"小时"+minute+"分"+second+"秒"; 
} 
window.setInterval(function(){ShowCountDown(2018,1,1,'ddyy');}, interval);
</script>