public RegularTimePeriod previous() {
        Month result;
        if (this.month != MonthConstants.JANUARY) {
            result = new Month(this.month - 1, this.year);
        }
        else {
            if (this.year > 1900) {
                result = new Month(MonthConstants.DECEMBER, this.year - 1);
            }
            else {
                result = null;
            }
        }
        return result;