def __sub__(self, *args, **kwargs):
    """x.__add__(y) <==> x-y."""
    r = super(BaseTimestamp, self).__sub__(*args, **kwargs)
    if isinstance(r, datetime.datetime):
      return type(self)(r.year, r.month, r.day, r.hour, r.minute, r.second,
                        r.microsecond, r.tzinfo)
    return r