FIX: robust DatetimeIndex.union across DST transitions

pandas/core/indexes/datetimelike.py

@@ -717,6 +717,35 @@ def _union(self, other, sort):
         assert isinstance(other, type(self))
         assert self.dtype == other.dtype
 
+        # For tz-aware DatetimeIndex, perform union in UTC to avoid
+        # local-time irregularities across DST transitions, then convert back.
+        tz = getattr(self.dtype, "tz", None)
+        if tz is not None:
+            other_tz = getattr(other.dtype, "tz", None)
+            if (
+                other_tz == tz
+                and isinstance(self._data, DatetimeArray)
+                and isinstance(other._data, DatetimeArray)
+            ):
+                left_utc_naive = self._data.tz_convert("UTC").tz_localize(None)
+                right_utc_naive = other._data.tz_convert("UTC").tz_localize(None)
+                left_naive = type(self)._simple_new(left_utc_naive, name=self.name)
+                right_naive = type(other)._simple_new(right_utc_naive, name=other.name)
+                res_naive = super(type(left_naive), left_naive)._union(
+                    right_naive, sort
+                )
+
+                if isinstance(res_naive, DatetimeArray):
+                    base_arr = res_naive
+                    name = self.name
+                else:
+                    base_arr = cast(DatetimeArray, res_naive._data)
+                    name = res_naive.name
+
+                res_arr = base_arr.tz_localize("UTC").tz_convert(tz)
+                res = type(self)._simple_new(res_arr, name=name)
+                return res._with_freq("infer")
+
         if self._can_range_setop(other):
             return self._range_union(other, sort=sort)
 

pandas/tests/indexes/datetimes/test_setops.py

@@ -60,6 +60,30 @@ def test_union3(self, sort, box):
         result = first.union(case, sort=sort)
         tm.assert_index_equal(result, expected)
 
+
+def test_union_across_dst_boundary():
+    # US/Eastern DST spring-forward on 2021-03-14 at 02:00
+    # (02:00-02:59 local time does not exist)
+    tz = "US/Eastern"
+    # Left side spans up to the missing hour window
+    left = date_range("2021-03-14 00:00", periods=3, freq="h", tz=tz)
+    # right side continues from the first valid post-DST hour
+    right = date_range("2021-03-14 03:00", periods=3, freq="h", tz=tz)
+
+    # Expect a union that preserves tz and includes valid hours without duplicates
+    expected = DatetimeIndex(
+        [
+            Timestamp("2021-03-14 00:00", tz=tz),
+            Timestamp("2021-03-14 01:00", tz=tz),
+            Timestamp("2021-03-14 03:00", tz=tz),
+            Timestamp("2021-03-14 04:00", tz=tz),
+            Timestamp("2021-03-14 05:00", tz=tz),
+        ]
+    ).as_unit(left.unit)
+
+    result = left.union(right)
+    tm.assert_index_equal(result, expected)
+
     @pytest.mark.parametrize("tz", tz)
     def test_union(self, tz, sort):
         rng1 = date_range("1/1/2000", freq="D", periods=5, tz=tz)