html部分

<h2>倒计时：（国庆节）</h2>

<span id="down"></span><br>

<button onclick="start()">start</button>

<button onclick="end()">stop</button><br>

js部分~~~~~~

<script>

//距离2017年还有

function start(){

str=setInterval("lick()",1000);

}

function lick()

{

var time=new Date();

var miao=time.getTime();

var times=new Date("Oct 1,2017 12:00:00");

var miao2=times.getTime();

var cutt=miao2-miao;

var sec=Math.ceil(cutt/1000%60);

var min=Math.ceil(cutt/1000/60%60);

var hour=Math.ceil(cutt/1000/60/60%24);

var day=Math.ceil(cutt/1000/60/60/24);

document.getElementById("down").innerHTML=day+"天"+hour+"小时"+min+"分"+sec+"秒";

document.getElementById("down").style.color="red";

}

function end(){

clearInterval(str);

}

<script>