js倒计时 天时分秒

function calcEndTime(endTime) {
      function num(n) {
        if (n < 0) {
          n = 0;
        }
        return n < 10 ? '0' + n : n;
      }
      let startTime = new Date();
      //算出中间差，以毫秒数返回.
      let countDown = endTime - startTime.getTime();
      //获取天数
      let oDay = Math.floor(countDown / 1000 / 60 / 60 / 24) ;
      //获取小时数
      let oHour = Math.floor((countDown / 1000 / 60 / 60) % 24);
      //获取分钟数
      let oMinute = Math.floor((countDown / 1000 / 60) % 60);
      //获取秒数
      let oSecond = Math.floor((countDown / 1000) % 60);
      //输出时间
      let t = '';
      if (oDay) {
        t += num(oDay) + '天';
      }
      t += num(oHour) + '时' + num(oMinute) + '分' + num(oSecond) + '秒';
      return t;
    }