添加span元素并更新其文本内容。

Add a span element and update its text content.

var span = document.getElementById('span');

function time() {
  var d = new Date();
  var s = d.getSeconds();
  var m = d.getMinutes();
  var h = d.getHours();
  span.textContent = h + ":" + m + ":" + s;
}

setInterval(time, 1000);

<span id="span"></span>