...

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      timerID: null,
      timer: 0
    };
  };

  componentDidMount() {
    console.log("Clock", "componentDidMount");
    const timerID = setInterval( this.updateTimer, 1000);
    this.setState( { timerID : timerID} );
  }

  componentWillUnmount() {
    console.log("Clock", "componentWillUnmount");
    clearInterval(this.state.timerID);
  }

  updateTimer = () => {
    this.setState( {timer: this.state.timer + 1 } );
  }


  render() {
    return (
      <div>
        <h1>Clock!</h1>

        <h3>{this.state.timer} s</h3>
      </div>
    );
  }
}