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() {
    let date = this.state.date;
    let year = date.getFullYear();
    let month = date.getMonth();
    let day = date.getDay();
    let seconds = date.getSeconds();

    return (
      <div>
        {year}年は{month}月の{day}日 {seconds}秒
      </div>
    );
  }
}
/*
ReactDOM.render(
  <Clock />,
  document.getElementById('clock')
);*/