use mean of min/max years as offset in calculation of datetime64 mean (#10035)

* use mean of min/max years as offset in caclulation of datetime64 mean

* reinstate _datetime_nanmin as it is used downstream in flox<0.10.0

* add whats-new.rst entry

* add whats-new.rst entry

Author: kmuehlbauer

doc/whats-new.rst

@@ -39,6 +39,9 @@ Bug fixes
   "nanoseconds" were chosen by default, which are optimal for
   nanosecond-resolution times, but not for times with coarser resolution. By
   `Spencer Clark <https://github.com/spencerkclark>`_ (:pull:`10017`).
+- Use mean of min/max years as offset in calculation of datetime64 mean
+  (:issue:`10019`, :pull:`10035`).
+  By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
 
 
 Documentation

xarray/core/duck_array_ops.py

@@ -550,7 +550,11 @@ def array_any(array, axis=None, keepdims=False, **kwargs):
 
 
 def _datetime_nanmin(array):
-    """nanmin() function for datetime64.
+    return _datetime_nanreduce(array, min)
+
+
+def _datetime_nanreduce(array, func):
+    """nanreduce() function for datetime64.
 
     Caveats that this function deals with:
 
@@ -562,7 +566,7 @@ def _datetime_nanmin(array):
     assert dtypes.is_datetime_like(dtype)
     # (NaT).astype(float) does not produce NaN...
     array = where(pandas_isnull(array), np.nan, array.astype(float))
-    array = min(array, skipna=True)
+    array = func(array, skipna=True)
     if isinstance(array, float):
         array = np.array(array)
     # ...but (NaN).astype("M8") does produce NaT
@@ -597,7 +601,7 @@ def datetime_to_numeric(array, offset=None, datetime_unit=None, dtype=float):
     # Set offset to minimum if not given
     if offset is None:
         if dtypes.is_datetime_like(array.dtype):
-            offset = _datetime_nanmin(array)
+            offset = _datetime_nanreduce(array, min)
         else:
             offset = min(array)
 
@@ -717,8 +721,11 @@ def mean(array, axis=None, skipna=None, **kwargs):
 
     array = asarray(array)
     if dtypes.is_datetime_like(array.dtype):
-        offset = _datetime_nanmin(array)
-
+        dmin = _datetime_nanreduce(array, min).astype("datetime64[Y]").astype(int)
+        dmax = _datetime_nanreduce(array, max).astype("datetime64[Y]").astype(int)
+        offset = (
+            np.array((dmin + dmax) // 2).astype("datetime64[Y]").astype(array.dtype)
+        )
         # From version 2025.01.2 xarray uses np.datetime64[unit], where unit
         # is one of "s", "ms", "us", "ns".
         # To not have to worry about the resolution, we just convert the output

xarray/tests/test_duck_array_ops.py

@@ -481,6 +481,19 @@ def test_cftime_datetime_mean(dask):
     assert_equal(result, expected)
 
 
+@pytest.mark.parametrize("dask", [False, True])
+def test_mean_over_long_spanning_datetime64(dask) -> None:
+    if dask and not has_dask:
+        pytest.skip("requires dask")
+    array = np.array(["1678-01-01", "NaT", "2260-01-01"], dtype="datetime64[ns]")
+    da = DataArray(array, dims=["time"])
+    if dask:
+        da = da.chunk({"time": 2})
+    expected = DataArray(np.array("1969-01-01", dtype="datetime64[ns]"))
+    result = da.mean()
+    assert_equal(result, expected)
+
+
 @requires_cftime
 @requires_dask
 def test_mean_over_non_time_dim_of_dataset_with_dask_backed_cftime_data():