def onOffset(self, date) -> bool:
        """Check if the given date is in the set of possible dates created
        using a length-one version of this offset class."""
        mod_month = (date.month - self.month) % 3
        return mod_month == 0 and date.day == self._get_offset_day(date)
Check if the given date is in the set of possible dates created
        using a length-one version of this offset class.
xarray/coding/cftime_offsets.py