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."""
        test_date = (self + date) - self
        return date == test_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