Un cronómetro en React

Cronometro.js

import React, {PropTypes} from 'react';
import Header from './Header';
import Screen from './Screen';
import Buttons from './Buttons';

export default class Cronometro extends React.Component {
    constructor() {
        super();

        this.timer = null;
        this.startedOn = 0;

        this.state = {
            elapsed: 0,
        };
        
        this.start = this.start.bind( this );
        this.stop = this.stop.bind( this );
    }
    start() {
        this.startedOn = new Date().getTime();
        this.timer = window.setInterval( this.updateTime.bind( this ), 55 );
    }
    updateTime() {
        this.setState( {elapsed: new Date().getTime() - this.startedOn } );
    }
    stop() {
        window.clearInterval( this.timer );
    }
    render() {
        return <div className="crono">
            <Header text="Cronómetro"/>
            <Screen elapsed={this.state.elapsed}/>
            <Buttons onStart={this.start} onStop={this.stop}/>
        </div>;
    }
}