BUG: Do not ignore sort in concat for DatetimeIndex

Author: rhshadrach

doc/source/whatsnew/v3.0.0.rst

@@ -370,6 +370,45 @@ In cases with mixed-resolution inputs, the highest resolution is used:
     In [2]: pd.to_datetime([pd.Timestamp("2024-03-22 11:43:01"), "2024-03-22 11:43:01.002"]).dtype
     Out[2]: dtype('<M8[ns]')
 
+.. _whatsnew_300.api_breaking.concat_datetime_sorting:
+
+:func:`concat` no longer ignores ``sort`` when all objects have a :class:`DatetimeIndex`
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When all objects passed to :func:`concat` have a :class:`DatetimeIndex`, the ``sort``
+argument is no longer ignored. Previously, the result would always be sorted along
+the non-concatenation axis even when ``sort=False`` (the default).
+
+.. ipython:: python
+
+    idx1 = pd.date_range("2025-01-02", periods=3, freq="h")
+    df1 = pd.DataFrame({"a": [1, 2, 3]}, index=idx1)
+    df1
+
+    idx2 = pd.date_range("2025-01-01", periods=3, freq="h")
+    df2 = pd.DataFrame({"b": [1, 2, 3]}, index=idx2)
+    df2
+
+*Old behavior*
+
+.. code-block:: ipython
+
+    In [3]: pd.concat([df1, df2], axis=1, sort=False)
+    Out[3]:
+                           a    b
+    2025-01-01 00:00:00  NaN  1.0
+    2025-01-01 01:00:00  NaN  2.0
+    2025-01-01 02:00:00  NaN  3.0
+    2025-01-02 00:00:00  1.0  NaN
+    2025-01-02 01:00:00  2.0  NaN
+    2025-01-02 02:00:00  3.0  NaN
+
+*New behavior*
+
+.. ipython:: python
+
+    pd.concat([df1, df2], axis=1, sort=False)
+
 .. _whatsnew_300.api_breaking.value_counts_sorting:
 
 Changed behavior in :meth:`DataFrame.value_counts` and :meth:`DataFrameGroupBy.value_counts` when ``sort=False``

pandas/core/indexes/api.py

@@ -227,7 +227,6 @@ def union_indexes(indexes, sort: bool | None = True) -> Index:
             raise TypeError("Cannot join tz-naive with tz-aware DatetimeIndex")
 
         if num_dtis == len(indexes):
-            sort = True
             result = indexes[0]
 
         elif num_dtis > 1:

pandas/tests/reshape/concat/test_datetimes.py

@@ -73,23 +73,23 @@ def test_concat_datetime_timezone(self):
 
         exp_idx = DatetimeIndex(
             [
-                "2010-12-31 15:00:00+00:00",
-                "2010-12-31 16:00:00+00:00",
-                "2010-12-31 17:00:00+00:00",
                 "2010-12-31 23:00:00+00:00",
                 "2011-01-01 00:00:00+00:00",
                 "2011-01-01 01:00:00+00:00",
+                "2010-12-31 15:00:00+00:00",
+                "2010-12-31 16:00:00+00:00",
+                "2010-12-31 17:00:00+00:00",
             ]
         ).as_unit("ns")
 
         expected = DataFrame(
             [
-                [np.nan, 1],
-                [np.nan, 2],
-                [np.nan, 3],
                 [1, np.nan],
                 [2, np.nan],
                 [3, np.nan],
+                [np.nan, 1],
+                [np.nan, 2],
+                [np.nan, 3],
             ],
             index=exp_idx,
             columns=["a", "b"],