Silence warning in maybe_promote

Author: spencerkclark

xarray/core/dtypes.py

@@ -88,7 +88,8 @@ def maybe_promote(dtype: T_dtype) -> tuple[T_dtype, Any]:
         # See https://github.com/numpy/numpy/issues/10685
         # np.timedelta64 is a subclass of np.integer
         # Check np.timedelta64 before np.integer
-        fill_value = np.timedelta64("NaT")
+        unit, _ = np.datetime_data(dtype)
+        fill_value = np.timedelta64("NaT", unit)
         dtype_ = dtype
     elif isdtype(dtype, "integral"):
         dtype_ = np.float32 if dtype.itemsize <= 2 else np.float64
@@ -97,8 +98,9 @@ def maybe_promote(dtype: T_dtype) -> tuple[T_dtype, Any]:
         dtype_ = dtype
         fill_value = np.nan + np.nan * 1j
     elif np.issubdtype(dtype, np.datetime64):
+        unit, _ = np.datetime_data(dtype)
         dtype_ = dtype
-        fill_value = np.datetime64("NaT")
+        fill_value = np.datetime64("NaT", unit)
     else:
         dtype_ = object
         fill_value = np.nan

xarray/tests/test_dtypes.py

@@ -102,8 +102,8 @@ def test_inf(obj) -> None:
         ("I", (np.float64, "nan")),  # dtype('uint32')
         ("l", (np.float64, "nan")),  # dtype('int64')
         ("L", (np.float64, "nan")),  # dtype('uint64')
-        ("m", (np.timedelta64, "NaT")),  # dtype('<m8')
-        ("M", (np.datetime64, "NaT")),  # dtype('<M8')
+        ("<m8[ns]", (np.dtype("<m8[ns]"), "NaT")),  # dtype('<m8[ns]')
+        ("<M8[ns]", (np.dtype("<M8[ns]"), "NaT")),  # dtype('<M8[ns]')
         ("O", (np.dtype("O"), "nan")),  # dtype('O')
         ("p", (np.float64, "nan")),  # dtype('int64')
         ("P", (np.float64, "nan")),  # dtype('uint64')