const oneMonthAgo =  moment().subtract(1, 'month').format('YYYY-MM-DD');
    console.log(oneMonthAgo)
    --> 2017-12-13
    console.log(typeof(oneMonthAgo))
    --> string

const timeSeries = this.props.stock_data["Monthly Time Series"];

    console.log(timeSeries["2017-12-13"]);
    --> Returns the right data

console.log(timeSeries[oneMonthAgo]);
   TypeError: undefined is not an object (evaluating 'timeSeries[oneMonthAgo]')