Timer.js

var Timer = React.createClass({

  getDefaultProps: function() {
    return {label: "Timer"}
  },

  getInitialState: function() {
    return {secondsElapsed: 0};
  },

  tick: function() {
    this.setState({secondsElapsed: this.state.secondsElapsed + 1});
  },

  componentDidMount: function() {
    this.interval = setInterval(this.tick, 1000);
  },

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

  render: function() {
    return (
      {this.props.label}
      Seconds Elapsed: {this.state.secondsElapsed}
    );
  }
});

React.render(<Timer label={"Sample timer"}/>, mountNode);