class MyComponent extends React.Component {

  componentDidMount() {
    this.isMounted = true;
  }

  componentWillUnmount() {
    this.isMounted = false;
  }

  render() {
    return (
      <div>
        {this.state.now}
      </div>
    );
  }

  updateTimeEverySecond() {
    setInterval(() => {
      if (this.isMounted) {
        this.setState({
          now: new Date().toISOString(),
        });
      }
    }, 1000);
  }
}