superjson/pkg/dateutil/tz/win.py
def picknthweekday(year, month, dayofweek, hour, minute, whichweek):
    """ dayofweek == 0 means Sunday, whichweek 5 means last instance """
    first = datetime.datetime(year, month, 1, hour, minute)

    # This will work if dayofweek is ISO weekday (1-7) or Microsoft-style (0-6),
    # Because 7 % 7 = 0
    weekdayone = first.replace(day=((dayofweek - first.isoweekday()) % 7) + 1)
    wd = weekdayone + ((whichweek - 1) * ONEWEEK)
    if (wd.month != month):
        wd -= ONEWEEK

    return wd
def picknthweekday(year, month, dayofweek, hour, minute, whichweek):
    """ dayofweek == 0 means Sunday, whichweek 5 means last instance """
    first = datetime.datetime(year, month, 1, hour, minute)

    # This will work if dayofweek is ISO weekday (1-7) or Microsoft-style (0-6),
    # Because 7 % 7 = 0
    weekdayone = first.replace(day=((dayofweek - first.isoweekday()) % 7) + 1)
    wd = weekdayone + ((whichweek - 1) * ONEWEEK)
    if (wd.month != month):
        wd -= ONEWEEK

    return wd
dayofweek == 0 means Sunday, whichweek 5 means last instance