<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<button>计时器</button>
<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>
var btn = document.getElementsByTagName("button")[0];
btn.onclick=function(){
var now = +new Date();
var EndTime= localStorage.setItem("starTime",now);
}
function GetRTime(){
var NowTime = new Date();
var starTime =localStorage.getItem("starTime");
var t =NowTime.getTime()-starTime;
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>
</body>
</html>
原文：http://www.cnblogs.com/zhaokuojiao/p/7603272.html