日期格式之——剩余时间（天，小时，分，秒）的计算方法

* { margin: 0;padding: 0;}
    #timer {
        font-size: 0px;color: red;font-weight: bolder;
        /*对字体进行抗锯齿渲染可以使字体看起来会更清晰舒服它有三个属性值:
        none: 对低像素的文本比较好;subpixel-antialiased:默认值;antialiased:抗锯齿很好 */
        -webkit-font-smoothing: antialiased;
    }
    span {font-size: 12px;}
    /*将span定宽，将重绘最小化*/
    #days,#hours,#minutes,#seconds {
        display: inline-block;
        width: 14px;
        text-align: center;
    }

<div id="timer">
         <span id="days"></span><span>天</span>
         <span id="hours"></span><span>小时</span>
         <span id="minutes"></span><span>分</span>
         <span id="seconds"></span><span>秒</span>
     </div>

var leftTime;
 function leftTimer(year,month,day,hour,minute,second){ 
  leftTime = (new Date(year,month-1,day,hour,minute,second)) - (new Date()); //计算剩余的毫秒数 
  var days = parseInt(leftTime / 1000 / 60 / 60 / 24 , 10); //计算剩余的天数 
  var hours = parseInt(leftTime / 1000 / 60 / 60 % 24 , 10); //计算剩余的小时 
  var minutes = parseInt(leftTime / 1000 / 60 % 60, 10);//计算剩余的分钟 
  var seconds = parseInt(leftTime / 1000 % 60, 10);//计算剩余的秒数 
  return {
    days: checkTime(days),
    hours: checkTime(hours),
    minutes: checkTime(minutes),
    seconds: checkTime(seconds) 
  }
}

var timer = setInterval(function(){ 
     var t = leftTimer(2017,11,21,12,0,0);
    if (!leftTime) clearInterval(timer);
     document.getElementById("days").innerHTML = t.days;
     document.getElementById("hours").innerHTML = t.hours;
     document.getElementById("minutes").innerHTML = t.minutes;
     document.getElementById("seconds").innerHTML = t.seconds;
 },1000);
 
function checkTime(i){ //将0-9的数字前面加上0，例1变为01 
  if(i<10) i = "0" + i; 
  return i; 
}