def to_datetimeindex(
        self, unsafe: bool = False, time_unit: Optional[PDDatetimeUnitOptions] = None
    ) -> pd.DatetimeIndex:
        """If possible, convert this index to a pandas.DatetimeIndex.

        Parameters
        ----------
        unsafe : bool
            Flag to turn off calendar mismatch warnings (default ``False``).
        time_unit : str
            Time resolution of resulting DatetimeIndex. Can be one of `"s"`,
            ``"ms"``, ``"us"``, or ``"ns"`` (default ``"ns"``).

        Returns
        -------
        pandas.DatetimeIndex

        Raises
        ------
        ValueError
            If the CFTimeIndex contains dates that are not possible in the
            standard calendar or outside the range representable by the
            specified ``time_unit``.

        Warns
        -----
        RuntimeWarning
            If converting from a non-standard calendar, or a Gregorian
            calendar with dates prior to the reform (1582-10-15).

        Warnings
        --------
        Note that for non-proleptic Gregorian calendars, this will change the
        calendar type of the index. In that case the result of this method
        should be used with caution.

        Examples
        --------
        >>> times = xr.date_range(
        ...     "2000", periods=2, calendar="gregorian", use_cftime=True
        ... )
        >>> times
        CFTimeIndex([2000-01-01 00:00:00, 2000-01-02 00:00:00],
                    dtype='object', length=2, calendar='standard', freq=None)
        >>> times.to_datetimeindex(time_unit="ns")
        DatetimeIndex(['2000-01-01', '2000-01-02'], dtype='datetime64[ns]', freq=None)
        """

        if not self._data.size:
            return pd.DatetimeIndex([])

        if time_unit is None:
            emit_user_level_warning(
                "In a future version of xarray to_datetimeindex will default "
                "to returning a 'us'-resolution DatetimeIndex instead of a "
                "'ns'-resolution DatetimeIndex. This warning can be silenced "
                "by explicitly passing the `time_unit` keyword argument.",
                FutureWarning,
            )
            time_unit = "ns"

        nptimes = cftime_to_nptime(self, time_unit=time_unit)
        calendar = infer_calendar_name(self)
        if calendar not in _STANDARD_CALENDARS and not unsafe:
            emit_user_level_warning(
                "Converting a CFTimeIndex with dates from a non-standard "
                f"calendar, {calendar!r}, to a pandas.DatetimeIndex, which "
                "uses dates from the standard calendar.  This may lead to "
                "subtle errors in operations that depend on the length of "
                "time between dates.",
                RuntimeWarning,
            )
        if calendar == "standard" and not unsafe:
            reform_date = self.date_type(1582, 10, 15)
            if self.min() < reform_date:
                emit_user_level_warning(
                    "Converting a CFTimeIndex with dates from a Gregorian "
                    "calendar that fall before the reform date of 1582-10-15 "
                    "to a pandas.DatetimeIndex. During this time period the "
                    "Gregorian calendar and the proleptic Gregorian calendar "
                    "of the DatetimeIndex do not exactly align. This warning "
                    "can be silenced by setting unsafe=True.",
                    RuntimeWarning,
                )

        return pd.DatetimeIndex(nptimes)
If possible, convert this index to a pandas.DatetimeIndex.

        Parameters
        ----------
        unsafe : bool
            Flag to turn off calendar mismatch warnings (default ``False``).
        time_unit : str
            Time resolution of resulting DatetimeIndex. Can be one of `"s"`,
            ``"ms"``, ``"us"``, or ``"ns"`` (default ``"ns"``).

        Returns
        -------
        pandas.DatetimeIndex

        Raises
        ------
        ValueError
            If the CFTimeIndex contains dates that are not possible in the
            standard calendar or outside the range representable by the
            specified ``time_unit``.

        Warns
        -----
        RuntimeWarning
            If converting from a non-standard calendar, or a Gregorian
            calendar with dates prior to the reform (1582-10-15).

        Warnings
        --------
        Note that for non-proleptic Gregorian calendars, this will change the
        calendar type of the index. In that case the result of this method
        should be used with caution.

        Examples
        --------
        >>> times = xr.date_range(
        ...     "2000", periods=2, calendar="gregorian", use_cftime=True
        ... )
        >>> times
        CFTimeIndex([2000-01-01 00:00:00, 2000-01-02 00:00:00],
                    dtype='object', length=2, calendar='standard', freq=None)
        >>> times.to_datetimeindex(time_unit="ns")
        DatetimeIndex(['2000-01-01', '2000-01-02'], dtype='datetime64[ns]', freq=None)
        
xarray/coding/cftimeindex.py