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That's how I've done it during React learning from official docs. You can see it live here: react-play-neone.herokuapp.com

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>
        <FormattedDate date={this.state.date} /> 
      </div>
    );
  }
}

Here is more info about arrow functions introduced in es6 es6-features.org