放学了倒计时

<script type="text/javascript">
    function task(){
        var now=new Date();
        var end=new Date('2018/5/25 18:00');
        var s=(end-now)/1000;
        if(s>0){
            var h=Math.floor(s/3600);
            var m=Math.floor(s%3600/60);
            s=Math.floor(s%60);
            document.querySelector('span').innerHTML=h+'小时'+m+'分钟'+s+'秒';
        }else{
            clearInterval(timer);
            document.querySelector('span').innerHTML='放学了';
        }
    }
        task();
        var timer=setInterval(task,1000);
        function stop(btn){
            if(btn.innerHTML=="||"){
                clearInterval(timer)
                btn.innerHTML="|>";
            }else{
                timer=setInterval(task,1000);
                btn.innerHTML="||";
            }
        }     
</script>