From 658fc1f517bfd1d335b9e031621a3d08ae77b3c5 Mon Sep 17 00:00:00 2001
From: Bert Palm <bert.palm@ufz.de>
Date: Tue, 12 Oct 2021 17:24:17 +0200
Subject: [PATCH 1/2] [FIX] for rolling with uneven nanoseconds have wrong
 results

---
 doc/source/whatsnew/v1.4.0.rst      |  1 +
 pandas/_libs/window/indexers.pyx    |  7 +++++++
 pandas/tests/window/test_rolling.py | 17 +++++++++++++++++
 3 files changed, 25 insertions(+)

diff --git a/doc/source/whatsnew/v1.4.0.rst b/doc/source/whatsnew/v1.4.0.rst
index a3dc95781ee93..0495280ac7ba0 100644
--- a/doc/source/whatsnew/v1.4.0.rst
+++ b/doc/source/whatsnew/v1.4.0.rst
@@ -505,6 +505,7 @@ Groupby/resample/rolling
 - Bug in :meth:`GroupBy.apply` with time-based :class:`Grouper` objects incorrectly raising ``ValueError`` in corner cases where the grouping vector contains a ``NaT`` (:issue:`43500`, :issue:`43515`)
 - Bug in :meth:`GroupBy.mean` failing with ``complex`` dtype (:issue:`43701`)
 - Fixed bug in :meth:`Series.rolling` and :meth:`DataFrame.rolling` not calculating window bounds correctly for the first row when ``center=True`` and index is decreasing (:issue:`43927`)
+- Fixed bug in :meth:`Series.rolling` and :meth:`DataFrame.rolling` for centered datetimelike windows with uneven nanosecond (:issue:`43997`)
 
 Reshaping
 ^^^^^^^^^
diff --git a/pandas/_libs/window/indexers.pyx b/pandas/_libs/window/indexers.pyx
index 3782b55bd19b3..64cb8bbdcf501 100644
--- a/pandas/_libs/window/indexers.pyx
+++ b/pandas/_libs/window/indexers.pyx
@@ -62,6 +62,13 @@ def calculate_variable_window_bounds(
     if closed in ['left', 'both']:
         left_closed = True
 
+    # If the forward and the backward facing windows
+    # would result in a fraction of 1/2 a nanosecond
+    # we need to make both interval ends inclusive.
+    if center and window_size % 2 == 1:
+        right_closed = True
+        left_closed = True
+
     if index[num_values - 1] < index[0]:
         index_growth_sign = -1
 
diff --git a/pandas/tests/window/test_rolling.py b/pandas/tests/window/test_rolling.py
index d88ce2ccb54cc..a0bf07156a65f 100644
--- a/pandas/tests/window/test_rolling.py
+++ b/pandas/tests/window/test_rolling.py
@@ -1243,6 +1243,23 @@ def test_rolling_decreasing_indices_centered(window, closed, expected, frame_or_
     tm.assert_equal(result_dec, expected_dec)
 
 
+@pytest.mark.parametrize(
+    "window,expected",
+    [
+        ("1ns", [1.0, 1.0, 1.0, 1.0]),
+        ("3ns", [2.0, 3.0, 3.0, 2.0]),
+    ],
+)
+def test_rolling_center_nanosecond_resolution(
+    window, closed, expected, frame_or_series
+):
+    index = date_range("2020", periods=4, freq="1ns")
+    df = frame_or_series([1, 1, 1, 1], index=index, dtype=float)
+    expected = frame_or_series(expected, index=index, dtype=float)
+    result = df.rolling(window, closed=closed, center=True).sum()
+    tm.assert_equal(result, expected)
+
+
 @pytest.mark.parametrize(
     "method,expected",
     [

From b6a6510065c3057f55f9636e456cdf646a42e976 Mon Sep 17 00:00:00 2001
From: rosagold <meinschallundrauch@gmail.com>
Date: Wed, 13 Oct 2021 12:25:35 +0200
Subject: [PATCH 2/2] Update indexers.pyx

added issue number in comment
---
 pandas/_libs/window/indexers.pyx | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pandas/_libs/window/indexers.pyx b/pandas/_libs/window/indexers.pyx
index 7d7342538a8d4..4b3a858ade773 100644
--- a/pandas/_libs/window/indexers.pyx
+++ b/pandas/_libs/window/indexers.pyx
@@ -62,6 +62,7 @@ def calculate_variable_window_bounds(
     if closed in ['left', 'both']:
         left_closed = True
 
+    # GH 43997:
     # If the forward and the backward facing windows
     # would result in a fraction of 1/2 a nanosecond
     # we need to make both interval ends inclusive.