__modifyDate(modify, { dateType, type }) {
    const tmpDate = new Date(this.centralDate);
    // if we're not working with days, reset
    // day count to first day of the month
    if (type !== 'Date') {
      tmpDate.setDate(1);
    }
    tmpDate[`set${type}`](tmpDate[`get${type}`]() + modify);
    // if we've reset the day count,
    // restore day count as best we can
    if (type !== 'Date') {
      const maxDays = new Date(tmpDate.getFullYear(), tmpDate.getMonth() + 1, 0).getDate();
      tmpDate.setDate(Math.min(this.centralDate.getDate(), maxDays));
    }
    this[dateType] = tmpDate;
  }
@param {number} modify
@param {Object} opts
@param {string} opts.dateType
@param {string} opts.type
@private