offsets.py 文件源码

def apply(self, other):
        n = self.n
        wkday, days_in_month = tslib.monthrange(other.year, self.month)

        first = _get_firstbday(wkday)

        years = n

        if n > 0:  # roll back first for positive n
            if (other.month < self.month or
                    (other.month == self.month and other.day < first)):
                years -= 1
        elif n <= 0:  # roll forward
            if (other.month > self.month or
                    (other.month == self.month and other.day > first)):
                years += 1

        # set first bday for result
        other = other + relativedelta(years=years)
        wkday, days_in_month = tslib.monthrange(other.year, self.month)
        first = _get_firstbday(wkday)
        return datetime(other.year, self.month, first, other.hour,
                        other.minute, other.second, other.microsecond)