add test for rolling max with DatetimeIndex

Author: ganevgv

pandas/tests/window/test_moments.py

@@ -1891,6 +1891,20 @@ def test_rolling_corr_with_zero_variance(self, window):
 
         assert s.rolling(window=window).corr(other=other).isna().all()
 
+    def test_rolling_max_datetimeindex(self):
+        # GH 21096
+        n = 10
+        index = pd.date_range(start="2018-1-1 01:00:00", freq="1min", periods=n)
+        s = Series(data=0, index=index)
+
+        s.iloc[1] = np.nan
+        s.iloc[-1] = 2
+        maxes = s.rolling(window=f"{n}min").max()
+        result = maxes.value_counts(dropna=False)
+        expected = Series(data={0.0: n - 1, 2.0: 1})
+
+        tm.assert_series_equal(result, expected)
+
     def _check_pairwise_moment(self, dispatch, name, **kwargs):
         def get_result(obj, obj2=None):
             return getattr(getattr(obj, dispatch)(**kwargs), name)(obj2)