倒记时

<div>距离15点还有[<span id="spanTime">00:00:00</span>]</div>

function addZero(value) {
                return value < 10 ? "0" + value : value;
            }

            function timeDiff() {
                var nowTime = new Date();
                var tarTime = new Date("2017/01/13 17:00:00");
                var spanTime = tarTime - nowTime;
                
                if(spanTime<=0){
                    window.clearInterval(timer);
                    return "活动结束了";
                }

                var hour = Math.floor(spanTime / (1000 * 60 * 60));  //计算出时间
                
                spanTime=spanTime-(hour*1000 * 60 * 60);  //减去小时
                var minute=Math.floor(spanTime/(1000*60)); //计算出分钟数
                
                spanTime=spanTime-(minute*1000*60);      //减去分钟
                var second=Math.floor(spanTime/1000);   //计算出秒数
                
                return addZero(hour)+":"+addZero(minute)+":"+addZero(second);
            }
            var spanTime=document.getElementById("spanTime");
            spanTime.innerHTML=timeDiff();
            
            var timer=window.setInterval(function(){
                spanTime.innerHTML=timeDiff();
            },1000)