import arrow


def diff_month(d1, d2):
    return (d1.year - d2.year) * 12 + d1.month - d2.month


def calc_deadline(start_date, period):
    date = arrow.get(start_date)
    now = arrow.get()
    remainder = diff_month(now, date) % period
    deadline = now.shift(months=period - 1 - remainder).datetime
    return deadline


if __name__ == '__main__':
    import datetime


    def return_date(func):
        def wrapper(*args):
            return func(*args).date()

        return wrapper


    calc_deadline = return_date(calc_deadline)

    # now is 2017-11-10
    assert calc_deadline(
        datetime.date(2017, 01, 01), 2) == datetime.date(2017, 12, 10)
    assert calc_deadline(
        datetime.date(2017, 01, 01), 1) == datetime.date(2017, 11, 10)
    assert calc_deadline(
        datetime.date(2017, 01, 01), 3) == datetime.date(2017, 12, 10)


    def calc_deadline():
        now = arrow.get('2017-01-31')
        print now.shift(months=1).shift(months=1)


    calc_deadline()