@classmethod
def start(cls, now, number, **options):
    """
        Return the starting datetime: ``number`` of months before ``now``.
        """
    year = now.year
    month = now.month - number + 1
    if month < 0:
        year = year + month // cls.MONTHS_IN_YEAR
        month = month % cls.MONTHS_IN_YEAR
    if month == 0:
        year = year - 1
        month = 12
    return cls.mask(now, **options).replace(year=year, month=month)
