diffDays = (startDate, endDate) => {
        const oneDayInMilliseconds = 24 * 60 * 60 * 1000;

        return Math.ceil(Math.abs((new Date(startDate).getTime() - new Date(endDate).getTime()) / (oneDayInMilliseconds)));
    }
Calculates difference between dates in days
@param  {String}  startDate  Date in string form
@param  {String}  endDate    Date in string form
@return {Number}             Number of days between dates