Js时间处理（时间差）

<div class="box">
    <div id="start"></div>
    <div id="love"></div>
  </div>
  <script>
    function phase(str, id) {
      const oneDay = 1000 * 60 * 60 * 24;
      const oneYear = oneDay * 365;
      const nowTime = Date.now()
      const startTime = new Date(str).getTime()
      let diff = nowTime - startTime;
      let surplus = Math.floor(diff / oneDay);
      let year = Math.floor(diff / oneYear);
      let mouth = Math.floor(diff / oneDay % 365 / 30);
      let day = Math.floor(diff / oneDay % 365 % 30);
      mouth < 10 ? '0' + mouth : mouth;
      day < 10 ? '0' + day : day;
      const start = document.getElementById(id);
      start.innerHTML = `<span class="y">${year}</span><span class="m">${mouth}</span><span class="d">${day}</span><em>${surplus}</em>`
    }
    phase('2011-12-29 00:00:00', 'start')
    phase('2012-07-17 00:00:00', 'love')

    let nDate = new Date(); //实例一个时间对象；
    nDate.getFullYear();   //获取系统的年；
    nDate.getMonth()+1;   //获取系统月份，由于月份是从0开始计算，所以要加1
    nDate.getDate(); // 获取系统日，
    nDate.getHours(); //获取系统时，
    nDate.getMinutes(); //分
    nDate.getSeconds(); //秒
  </script>