原理：定的时间 -现在时间=时间差
讲时间差转换成时 分 秒 显示出来
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>简单易用的倒计时js代码 - 网站源码</title>
<style>
*{ margin:0; padding:0; list-style:none;}
body{ font-size:18px; text-align:center;}
.time{ height:30px; padding:200px;}
</style>
</head>
<body>
<div class="time">
<span id="t_d">00天</span>
<span id="t_h">00时</span>
<span id="t_m">00分</span>
<span id="t_s">00秒</span>
</div>
<script>
function GetRTime(){
var EndTime= new Date('2019/06/20 00:00:00');
var NowTime = new Date();
var t =EndTime.getTime() - NowTime.getTime();
var d=Math.floor(t/1000/60/60/24);
var h=Math.floor(t/1000/60/60%24);
var m=Math.floor(t/1000/60%60);
var s=Math.floor(t/1000%60);
document.getElementById("t_d").innerHTML = d + "天";
document.getElementById("t_h").innerHTML = h + "时";
document.getElementById("t_m").innerHTML = m + "分";
document.getElementById("t_s").innerHTML = s + "秒";
}
setInterval(GetRTime,0);
</script>
<div style="text-align:center;margin:50px 0; font:normal 14px/24px 'MicroSoft YaHei';">
</div>
</body>
</html>
每秒刷新一下