`<style>
 div{
   height:50px;
   line-height:50px;
   text-align:center;
   border:double #ccc;
   width:100px;
 }

</style>
<script type="text/javascript">
    var t ;//注意下，这个t必须要是全局变量
    var flag = true;
    function contral(){
        flag ? startDate():clearTimeout(t);
        flag = !flag;
    }
    function startDate(){
        var date = new Date();
        var h = date.getHours();
        var m = date.getMinutes();
        var s = date.getSeconds();

            //在这里要对分和秒做判断
        m = checkDate(m);
        s = checkDate(s);

        document.getElementById('clock').innerHTML = h+':'+m+':'+s;

        t = setTimeout('startDate()',1000);

        return t;
    }
    function checkDate(i){
        if(i<10)    return '0'+i;
        return i;
    }

</script>

<div id="clock"></div>
    <button onclick="contral()">开始/暂停</button>
<script>

</script>
`