ReactJS learning. Clock using tock js class

clock.jsx

// Clock Component
var timer;
var Clock = React.createClass({
  componentDidMount: function() {
    var self = this;
    timer = new Tock({
      countdown: false,
      interval: 10,
      callback: function() {
        self.setState({
          time: timer.msToTime(timer.lap())
        })
      }
    });
  },
  getInitialState: function() {
    return {
      time: ''
    }
  },
  render: function() {
    return (
      <div>
        <label>{this.state.time}</label>
        <br />
        <button type="button" onClick={this.start}>Start</button>
        <button type="button" onClick={this.pause}>Pause</button>
        <button type="button" onClick={this.reset}>Reset</button>
      </div>);
  },
  start: function(){
    timer.start(timer.lap());
  },
  pause: function(){
    timer.pause();
  },
  reset: function(){
    timer.start(0);
  }
});

React.render(<Clock /> , document.getElementById('content'));