React Timer

var TimerExample = React.createClass({

  getInitialState: function () {
    return {elapsed: 0}
  },

  componentDidMount: function () {
    this.timer = setInterval(this.tick, 50);
  },

  componentWillUnmount: function() {
    clearInterval(this.timer);
  },

  tick: function () {
    this.setState({elapsed: new Date() - this.props.start}
    );
  },
  render: function () {
    var elapsed = Math.round(this.state.elapsed / 100);
    var seconds = (elapsed / 10).toFixed(1);
    return <p>{seconds}</p>
  }

});

ReactDOM.render(
  <TimerExample start={Date.now()}/>,
  document.getElementById('container')
);