I have a basic ES6 react app and am trying to use momentJS to manipulate some dates. For some reason I keep getting `month.add is not a function`

Currently the code I have is this: 

```
export default class CalendarApp extends React.Component {

constructor() {
    super();

    this.state = {
        currentDate: Moment().format(),
        month: Moment().format('MMMM'),
        year: Moment().format('YYYY')
    }

    // Bind Methods to this
    this.previousMonth = this.previousMonth.bind(this);
    this.nextMonth = this.nextMonth.bind(this);
}

previousMonth() {
    let month = this.state.month;
    month.add(-1, 'months');
    this.setState({
        month: month
    })
}

nextMonth() {
    let month = this.state.month;
    month.add(1, 'months');
    this.setState({
        month: month
    })
}

render() {
    return (
        <div className="calendar">
            <div className="calendar-container" style={ hideCalendar }>
                <caption>
                    <button className="previous-month" onClick={ this.previousMonth }>Prev</button>
                    <button className="next-month" onClick={ this.nextMonth }>Next</button> 
                    <div className="calendar-month">
                        <span>{ this.state.month } { this.state.year }</span>
                    </div>
                </caption>
            </div>
        </div>
    )
}

```

}

I have tried various versions of setting the initial state using `Moment().month()` etc but nothing seems to work. Any help would be much appreciated.