React.js: clock class

<!DOCTYPE html>
<html>

  <head>
    <script data-require="jquery@*" data-semver="3.0.0" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.0.0/jquery.js"></script>
    <script src="https://npmcdn.com/react@15.4/dist/react.js"></script>
    <script src="https://npmcdn.com/react-dom@15.4/dist/react-dom.js"></script>
    <link rel="stylesheet" href="style.css" />
  </head>

  <body>
    <div id="root"></div>
    <script src="script.js"></script>
  </body>

</html>

"use babel";

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {date: new Date()};
  }
  componentDidMount(){
    this.timerId = setInterval(
      () => this.tick(),
      1000
    );
  }
  componentWillUnmount() {
    clearInterval(this.timerId);
  }
  tick() {
    this.setState({
      date: new Date()
    });
  }
  render() {
    return (
      <div>
        <h1>Hello, World!</h1>
        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}

ReactDOM.render(
  <Clock />,
  document.getElementById('root')
);