符号無し整数で表現された日付に対して指定した月数や日数を引き算／足し算して返す

class date_as_integer:
    """符号無し整数で表現された日付に対する演算子をまとめたクラス
    A class of operators for dates represented by unsigned integers
    """

    @staticmethod
    def add_days(orig_date, duration):
        """8か6桁の符号無し整数（yyyyMMdd か yyMMdd）で表現された
        日付について、指定した日数を加算、または減算した結果を返す関数
        A function implementation that returns the result of
        adding or subtracting the specified number of days for a date
        represented by an 8- or 6-digit unsigned integer (yyyyMMdd or yyMMdd).
        """
        Y, _md = divmod(orig_date, 10000)
        M, D = divmod(_md, 100)
        M -= 1
        D -= 1

        def get_ndays_month(year):

            months_of_year = [
                [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31], # not leap
                [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]  # leap year
            ]

            return months_of_year[
                year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
            ]

        def future(ndays, duration, Y, M):
            ndays = get_ndays_month(Y)[M]
            M += 1
            duration -= ndays
            if M > 11:
                M = 0
                Y += 1
            return ndays, duration, Y, M

        def past(ndays, duration, Y, M):
            M -= 1
            ndays = get_ndays_month(Y)[M]
            duration += ndays
            if M < 0:
                M = 11
                Y -= 1
            return ndays, duration, Y, M

        step_into_next_month = future if (duration >= 0) else past

        ndays = get_ndays_month(Y)[M]

        while not ndays > (D + duration) >= 0:

            ndays, duration, Y, M = step_into_next_month(ndays, duration, Y, M)

        D += duration

        return Y * 10000 + (M + 1) * 100 + D + 1


    @staticmethod
    def add_months(orig_date, duration):
        """6か4桁の符号無し整数（yyyyMM か yyMM）で表現された
        日付について、指定した月数を加算、または減算した結果を返す関数
        A function implementation that returns the result of
        adding or subtracting the specified number of months for a date
        represented by an 6- or 4-digit unsigned integer (yyyyMM or yyMM).
        """
        Y0, M0 = divmod(int(orig_date), 100)
        dy, dm = divmod(duration, 12)

        if M0 + dm > 12:
            Y1 = Y0 + dy + 1
            M1 = M0 + dm - 12
        else:
            Y1 = Y0 + dy
            M1 = M0 + dm

        return Y1 * 100 + M1


if __name__ == "__main__":
    print('Up and Down at new year boundary')
    for i in range(2):
        print(date_as_integer.add_days(20211231, i))
    for i in range(2):
        print(date_as_integer.add_days(20220101, -i))

    print('At boundary of leap day every 4 years')
    for i in range(3):
        print(date_as_integer.add_days(20200228, i))
    for i in range(3):
        print(date_as_integer.add_days(20200301, -i))

    print('''At boundary of leap day in a year that cannot divide by 100
(28 days are in February) ''')
    for i in range(2):
        print(date_as_integer.add_days(21000228, i))
    for i in range(2):
        print(date_as_integer.add_days(21000301, -i))

    print('''At boundary of leap day in a year that can divide by 400
(29 days are in February) ''')
    for i in range(3):
        print(date_as_integer.add_days(20000228, i))
    for i in range(3):
        print(date_as_integer.add_days(20000301, -i))

    print('Find the date N months later (or earlier)')
    for i in range(2):
        print(date_as_integer.add_months(200012, i))
    for i in range(2):
        print(date_as_integer.add_months(200101, -i))

    print('38 == 6 + 12 + 12 + 8')
    print(date_as_integer.add_months(200006, 38))
    print(date_as_integer.add_months(200006, -38))