js倒计时代码，很有用，可设置任意时间进行倒计时显示

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
距离 <span id="endtime" style="color:#0000FF;"></span> 剩余时间：<span id="rest" style="color:#FF0000;"></span>
<script LANGUAGE="javascript">
<!--
//显示时间函数
function showtime() {
      Today = new Date();
      var NowYear = Today.getYear();
      var NowMonth = Today.getMonth();
      var NowDate = Today.getDate();
      var NowHour = Today.getHours();
      var NowMinute = Today.getMinutes();
      var NowSecond = Today.getSeconds();
      if (NowYear <2000) NowYear=1900+NowYear;
      Today = null;
      Yearleft = thisyear - NowYear
      Monthleft = thisMonth - NowMonth
      Dateleft = thisday - NowDate
      Hourleft = thisHours - NowHour
      Minuteleft = thisminute - NowMinute
      Secondleft = thisSecond - NowSecond
      if (Secondleft<0){
            Secondleft=60+Secondleft;
            Minuteleft=Minuteleft-1;
      }
      if (Minuteleft<0){
            Minuteleft=60+Minuteleft;
            Hourleft=Hourleft-1;
      }
      if (Hourleft<0){
            Hourleft=24+Hourleft;
            Dateleft=Dateleft-1;
      }
      if (Dateleft<0){
            Dateleft=31+Dateleft;
            Monthleft=Monthleft-1;
      }
      if (Monthleft<0){
            Monthleft=12+Monthleft;
            Yearleft=Yearleft-1;
      }
      //这里是要显示的值,根据你的需要而控制要显示的年月日时分秒
      Temp=Yearleft + ' 年 '+ Monthleft + ' 月 '+Dateleft+' 天 '+Hourleft+' 小时 '+Minuteleft+' 分 '+Secondleft+' 秒 ';
      if (Yearleft<0){//当倒计时为0时要处理的程序
            //在此加入时间倒计时至0时要处理的程序
            document.getElementById("rest").innerHTML="结束";
      }else{ //当倒计时还没有到0时处理的程序,在此是显示时间
            document.getElementById("rest").innerHTML=Temp;
      }
      timerID = setTimeout("showtime()",1000);//每一秒钟运行一次函数
      timerRunning = true;
}


//停止计时函数
function stopclock() {
      if(timerRunning)
      clearTimeout(timerID);
      timerRunning = false;
}

//开始计时函数
function startclock()
{
      stopclock();
      showtime();
}

//////////////////////////////////////////////////////////////////
// 调用函数
//////////////////////////////////////////////////////////////////
var timerID = null;
var timerRunning = false;
//设置当前用于倒计时的日期时间
Today = new Date();
var thisyear= 2020; //设置年，或使用Today.getFullYear()
var thisMonth= 12; //设置月，或使用Today.getMonth()
var thisday= 30; //设置日，或使用Today.getDate()
var thisHours= 18; //设置时，或使用Today.getHours()
var thisminute= 0; //设置分，或使用Today.getMinutes()
var thisSecond = 0; //设置秒，或使用Today.getSeconds()

document.getElementById("endtime").innerHTML = thisyear + "-" + thisMonth + "-" + thisday + " " + thisHours + ":" + thisminute + ":" + thisSecond + " ";
startclock();//调用函数
-->
</script>