BUG: Fixed inconsistent offset behavior for series #43784

doc/source/whatsnew/v1.5.0.rst

@@ -992,8 +992,9 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.resolution` incorrectly returning "day" instead of "nanosecond" for nanosecond-resolution indexes (:issue:`46903`)
 - Bug in :class:`Timestamp` with an integer or float value and ``unit="Y"`` or ``unit="M"`` giving slightly-wrong results (:issue:`47266`)
 - Bug in :class:`.DatetimeArray` construction when passed another :class:`.DatetimeArray` and ``freq=None`` incorrectly inferring the freq from the given array (:issue:`47296`)
-- Bug in :func:`to_datetime` where ``OutOfBoundsDatetime`` would be thrown even if ``errors=coerce`` if there were more than 50 rows (:issue:`45319`)
 - Bug when adding a :class:`DateOffset` to a :class:`Series` would not add the ``nanoseconds`` field (:issue:`47856`)
+- Bug where adding a :class:`DateOffset` to a :class:`DatetimeIndex` or :class:`Series` over a Daylight Savings Time boundary would produce an incorrect result (:issue:`43784`)
+- Bug in :func:`to_datetime` where ``OutOfBoundsDatetime`` would be thrown even if ``errors=coerce`` if there were more than 50 rows (:issue:`45319`)
 -
 
 Timedelta

pandas/core/arrays/datetimes.py

@@ -40,6 +40,7 @@
     tz_convert_from_utc,
     tzconversion,
 )
+from pandas._libs.tslibs.offsets import DateOffset
 from pandas._typing import npt
 from pandas.errors import (
     OutOfBoundsDatetime,
@@ -692,7 +693,10 @@ def _add_offset(self, offset) -> DatetimeArray:
         assert not isinstance(offset, Tick)
 
         if self.tz is not None:
-            values = self.tz_localize(None)
+            if not offset._use_relativedelta and type(offset) == DateOffset:
+                values = self.tz_convert("utc").tz_localize(None)
+            else:
+                values = self.tz_localize(None)
         else:
             values = self
 
@@ -714,7 +718,10 @@ def _add_offset(self, offset) -> DatetimeArray:
             result = DatetimeArray._simple_new(result, dtype=result.dtype)
             if self.tz is not None:
                 # FIXME: tz_localize with non-nano
-                result = result.tz_localize(self.tz)
+                if not offset._use_relativedelta and type(offset) == DateOffset:
+                    result = result.tz_localize("utc").tz_convert(self.tz)
+                else:
+                    result = result.tz_localize(self.tz)
 
         return result
 

pandas/tests/tseries/offsets/test_dst.py

@@ -30,6 +30,8 @@
     YearEnd,
 )
 
+import pandas as pd
+import pandas._testing as tm
 from pandas.tests.tseries.offsets.test_offsets import get_utc_offset_hours
 from pandas.util.version import Version
 
@@ -228,3 +230,27 @@ def test_nontick_offset_with_ambiguous_time_error(original_dt, target_dt, offset
     msg = f"Cannot infer dst time from {target_dt}, try using the 'ambiguous' argument"
     with pytest.raises(pytz.AmbiguousTimeError, match=msg):
         localized_dt + offset
+
+
+def test_series_dst_addition():
+    # GH#43784
+    startdates = pd.Series(
+        [
+            Timestamp("2020-10-25", tz="Europe/Berlin"),
+            Timestamp("2017-03-12", tz="US/Pacific"),
+        ]
+    )
+    offset1 = DateOffset(hours=3)
+    offset2 = DateOffset(days=1)
+
+    expected1 = pd.Series(
+        [Timestamp("2020-10-25 02:00:00+01:00"), Timestamp("2017-03-12 04:00:00-07:00")]
+    )
+
+    expected2 = pd.Series(
+        [Timestamp("2020-10-26 00:00:00+01:00"), Timestamp("2017-03-13 00:00:00-07:00")]
+    )
+
+    tm.assert_series_equal((startdates + offset1), expected1)
+
+    tm.assert_series_equal((startdates + offset2), expected2)