<script>
  (function () {

    var tian = document.getElementsByClassName('JS-tian')[0];
    var shi = document.getElementsByClassName('JS-shi')[0];
    var fen = document.getElementsByClassName('JS-fen')[0];
    var miao = document.getElementsByClassName('JS-miao')[0];

    var endTime = new Date('2117/07/12 23:59:59').getTime() + 1000;

    var interval = null;
    interval = setInterval(function () {
      var syhm = endTime - Date.now(); // 剩余毫秒
      if (syhm >= 0) {
        tian.innerText = Math.floor(syhm / 1000 / 60 / 60 / 24);
        shi.innerText = Math.floor(syhm / 1000 / 60 / 60 % 24);
        fen.innerText = Math.floor(syhm / 1000 / 60 % 60);
        miao.innerText = Math.floor(syhm / 1000 % 60);
      } else {
        clearInterval(interval);
      }
    }, 0);

  })();
</script>

<div>
  距结束还剩：<span class="JS-tian"></span>天<span class="JS-shi"></span>时<span class="JS-fen"></span>分<span class="JS-miao"></span>秒
</div>