test.js

var Timer = React.createClass({
    getInitialState: function() {
        return {
            secondsElapsed: Number(this.props.startTime) || 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 (
            <div>
                Seconds Elapsed: {this.state.secondsElapsed}
            </div>
        );
    }
});
ReactDOM.render(< Timer startTime = "60" / >, app);