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>The time is {this.state.date.toLocaleTimeString()}</h2>
        </div>);
    }
}

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