new Date().toLocaleTimeString()

function tick() {
  const element = (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
    </div>
  );
  ReactDOM.render(
    element,
    document.getElementById('root')
  );
}

setInterval(tick, 1000);

new Date().toLocaleTimeString() 返回本地时间字符串

所有Date（）对象方法参考：

http://www.runoob.com/jsref/jsref-obj-date.html