Fix infer_freq, check for subdtype "datetime64"/"timedelta64" (#9999)

* Fix infer_freq, check for subdtype "datetime64"/"timedelta64"

* update infer_freq test

* add whats-new.rst entry

* add typing to test function

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: kmuehlbauer

doc/whats-new.rst

@@ -63,7 +63,7 @@ eventually be deprecated.
 
 New Features
 ~~~~~~~~~~~~
-- Relax nanosecond datetime / timedelta restriction in CF time decoding (:issue:`7493`, :pull:`9618`, :pull:`9966`, :pull:`9977`).
+- Relax nanosecond datetime restriction in CF time decoding (:issue:`7493`, :pull:`9618`, :pull:`9977`, :pull:`9966`, :pull:`9999`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_ and `Spencer Clark <https://github.com/spencerkclark>`_.
 - Enable the ``compute=False`` option in :py:meth:`DataTree.to_zarr`. (:pull:`9958`).
   By `Sam Levang <https://github.com/slevang>`_.

xarray/coding/frequencies.py

@@ -48,6 +48,7 @@
 from xarray.coding.cftime_offsets import _MONTH_ABBREVIATIONS, _legacy_to_new_freq
 from xarray.coding.cftimeindex import CFTimeIndex
 from xarray.core.common import _contains_datetime_like_objects
+from xarray.core.dtypes import _is_numpy_subdtype
 
 _ONE_MICRO = 1
 _ONE_MILLI = _ONE_MICRO * 1000
@@ -88,9 +89,10 @@ def infer_freq(index):
         elif not _contains_datetime_like_objects(Variable("dim", index)):
             raise ValueError("'index' must contain datetime-like objects")
         dtype = np.asarray(index).dtype
-        if dtype == "datetime64[ns]":
+
+        if _is_numpy_subdtype(dtype, "datetime64"):
             index = pd.DatetimeIndex(index.values)
-        elif dtype == "timedelta64[ns]":
+        elif _is_numpy_subdtype(dtype, "timedelta64"):
             index = pd.TimedeltaIndex(index.values)
         else:
             index = CFTimeIndex(index.values)

xarray/tests/test_cftimeindex.py

@@ -19,6 +19,7 @@
     _parse_iso8601,
     parse_iso8601_like,
 )
+from xarray.core.types import PDDatetimeUnitOptions
 from xarray.tests import (
     assert_array_equal,
     assert_identical,
@@ -1393,16 +1394,16 @@ def test_asi8_empty_cftimeindex():
 
 
 @requires_cftime
-def test_infer_freq_valid_types():
+def test_infer_freq_valid_types(time_unit: PDDatetimeUnitOptions) -> None:
     cf_indx = xr.cftime_range("2000-01-01", periods=3, freq="D")
     assert xr.infer_freq(cf_indx) == "D"
     assert xr.infer_freq(xr.DataArray(cf_indx)) == "D"
 
-    pd_indx = pd.date_range("2000-01-01", periods=3, freq="D")
+    pd_indx = pd.date_range("2000-01-01", periods=3, freq="D").as_unit(time_unit)
     assert xr.infer_freq(pd_indx) == "D"
     assert xr.infer_freq(xr.DataArray(pd_indx)) == "D"
 
-    pd_td_indx = pd.timedelta_range(start="1D", periods=3, freq="D")
+    pd_td_indx = pd.timedelta_range(start="1D", periods=3, freq="D").as_unit(time_unit)
     assert xr.infer_freq(pd_td_indx) == "D"
     assert xr.infer_freq(xr.DataArray(pd_td_indx)) == "D"