Updating and re-opening PR #48129

doc/source/whatsnew/v2.0.0.rst

@@ -819,6 +819,7 @@ Datetimelike
 - Bug in :func:`to_datetime` was throwing ``ValueError`` when parsing dates with ISO8601 format where some values were not zero-padded (:issue:`21422`)
 - Bug in :func:`to_datetime` was giving incorrect results when using ``format='%Y%m%d'`` and ``errors='ignore'`` (:issue:`26493`)
 - Bug in :func:`to_datetime` was failing to parse date strings ``'today'`` and ``'now'`` if ``format`` was not ISO8601 (:issue:`50359`)
+- 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`)
 -
 
 Timedelta

pandas/_libs/tslibs/offsets.pyx

@@ -1093,6 +1093,7 @@ cdef class Day(Tick):
     _td64_unit = "D"
     _period_dtype_code = PeriodDtypeCode.D
     _creso = NPY_DATETIMEUNIT.NPY_FR_D
+    _use_relativedelta = True
 
 
 cdef class Hour(Tick):
@@ -1439,6 +1440,7 @@ cdef class BusinessMixin(SingleConstructorOffset):
     """
     Mixin to business types to provide related functions.
     """
+    _use_relativedelta = True
 
     cdef readonly:
         timedelta _offset
@@ -2062,6 +2064,7 @@ cdef class WeekOfMonthMixin(SingleConstructorOffset):
     """
     Mixin for methods common to WeekOfMonth and LastWeekOfMonth.
     """
+    _use_relativedelta = True
 
     cdef readonly:
         int weekday, week
@@ -2106,6 +2109,7 @@ cdef class YearOffset(SingleConstructorOffset):
     DateOffset that just needs a month.
     """
     _attributes = tuple(["n", "normalize", "month"])
+    _use_relativedelta = True
 
     # FIXME(cython#4446): python annotation here gives compile-time errors
     # _default_month: int
@@ -2271,6 +2275,7 @@ cdef class QuarterOffset(SingleConstructorOffset):
     # FIXME(cython#4446): python annotation here gives compile-time errors
     # _default_starting_month: int
     # _from_name_starting_month: int
+    _use_relativedelta = True
 
     cdef readonly:
         int startingMonth
@@ -2447,6 +2452,8 @@ cdef class QuarterBegin(QuarterOffset):
 # Month-Based Offset Classes
 
 cdef class MonthOffset(SingleConstructorOffset):
+    _use_relativedelta = True
+
     def is_on_offset(self, dt: datetime) -> bool:
         if self.normalize and not _is_normalized(dt):
             return False
@@ -2573,6 +2580,7 @@ cdef class SemiMonthOffset(SingleConstructorOffset):
     _default_day_of_month = 15
     _min_day_of_month = 2
     _attributes = tuple(["n", "normalize", "day_of_month"])
+    _use_relativedelta = True
 
     cdef readonly:
         int day_of_month
@@ -2790,6 +2798,7 @@ cdef class Week(SingleConstructorOffset):
     _inc = timedelta(weeks=1)
     _prefix = "W"
     _attributes = tuple(["n", "normalize", "weekday"])
+    _use_relativedelta = True
 
     cdef readonly:
         object weekday  # int or None
@@ -3069,6 +3078,7 @@ cdef class LastWeekOfMonth(WeekOfMonthMixin):
 # Special Offset Classes
 
 cdef class FY5253Mixin(SingleConstructorOffset):
+    _use_relativedelta = True
     cdef readonly:
         int startingMonth
         int weekday
@@ -3540,6 +3550,7 @@ cdef class Easter(SingleConstructorOffset):
     >>> ts + pd.offsets.Easter()
     Timestamp('2022-04-17 00:00:00')
     """
+    _use_relativedelta = True
 
     cpdef __setstate__(self, state):
         self.n = state.pop("n")

pandas/core/arrays/datetimes.py

@@ -761,7 +761,10 @@ 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:
+                values = self.tz_convert("utc").tz_localize(None)
+            else:
+                values = self.tz_localize(None)
         else:
             values = self
 
@@ -782,7 +785,10 @@ def _add_offset(self, offset) -> DatetimeArray:
         else:
             result = DatetimeArray._simple_new(result, dtype=result.dtype)
             if self.tz is not None:
-                result = result.tz_localize(self.tz)
+                if not offset._use_relativedelta:
+                    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.util.version import Version
 
 # error: Module has no attribute "__version__"
@@ -233,3 +235,30 @@ 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")]
+    )
+
+    result1 = startdates + offset1
+    result2 = startdates + offset2
+
+    tm.assert_series_equal(result1, expected1)
+
+    tm.assert_series_equal(result2, expected2)