function isInPreviousMonth(startDate, endDate) {
      var previousMonth = getDateInPreviousMonth(startDate);
      return isSameMonthAndYear(endDate, previousMonth);
    }
Gets whether a date is in the month immediately before some date.
@param {Date} startDate The date from which to compare.
@param {Date} endDate The date to check.
@returns {boolean}