BUG: honor ambiguous/nonexistent for tz-aware endpoints in date_range (#62493)

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

Author: floura-angel

doc/source/whatsnew/v3.0.0.rst

@@ -981,7 +981,8 @@ Timezones
 ^^^^^^^^^
 - Bug in :meth:`DatetimeIndex.union`, :meth:`DatetimeIndex.intersection`, and :meth:`DatetimeIndex.symmetric_difference` changing timezone to UTC when merging two DatetimeIndex objects with the same timezone but different units (:issue:`60080`)
 - Bug in :meth:`Series.dt.tz_localize` with a timezone-aware :class:`ArrowDtype` incorrectly converting to UTC when ``tz=None`` (:issue:`61780`)
--
+- Fixed bug in :func:`date_range` where tz-aware endpoints with calendar offsets (e.g. ``"MS"``) failed on DST fall-back. These now respect ``ambiguous``/ ``nonexistent``. (:issue:`52908`)
+
 
 Numeric
 ^^^^^^^

pandas/core/arrays/datetimes.py

@@ -456,13 +456,14 @@ def _generate_range(
             end = _maybe_localize_point(end, freq, tz, ambiguous, nonexistent)
 
         if freq is not None:
-            # We break Day arithmetic (fixed 24 hour) here and opt for
-            # Day to mean calendar day (23/24/25 hour). Therefore, strip
-            # tz info from start and day to avoid DST arithmetic
-            if isinstance(freq, Day):
-                if start is not None:
+            # Offset handling:
+            # Ticks (fixed-duration like hours/minutes): keep tz; do absolute-time math.
+            # Other calendar offsets: drop tz; do naive wall time; localize once later
+            # so `ambiguous`/`nonexistent` are applied correctly.
+            if not isinstance(freq, Tick):
+                if start is not None and start.tz is not None:
                     start = start.tz_localize(None)
-                if end is not None:
+                if end is not None and end.tz is not None:
                     end = end.tz_localize(None)
 
             if isinstance(freq, (Tick, Day)):

pandas/tests/indexes/datetimes/test_date_range.py

@@ -1740,3 +1740,28 @@ def test_date_range_negative_freq_year_end_inbounds(self, unit):
             freq="-1YE",
         )
         tm.assert_index_equal(rng, exp)
+
+    def test_date_range_tzaware_endpoints_accept_ambiguous(self):
+        # https://github.com/pandas-dev/pandas/issues/52908
+        start = Timestamp("1916-08-01", tz="Europe/Oslo")
+        end = Timestamp("1916-12-01", tz="Europe/Oslo")
+        res = date_range(start, end, freq="MS", ambiguous=True)
+        exp = date_range(
+            "1916-08-01", "1916-12-01", freq="MS", tz="Europe/Oslo", ambiguous=True
+        )
+        tm.assert_index_equal(res, exp)
+
+    def test_date_range_tzaware_endpoints_accept_nonexistent(self):
+        # Europe/London spring-forward: 2015-03-29 01:30 does not exist.
+        tz = "Europe/London"
+        start = Timestamp("2015-03-28 01:30", tz=tz)
+        end = Timestamp("2015-03-30 01:30", tz=tz)
+
+        result = date_range(start, end, freq="D", nonexistent="shift_forward")
+
+        # Build expected by generating naive daily times, then tz_localize so
+        # the nonexistent handling is applied during localization.
+        expected = date_range(
+            "2015-03-28 01:30", "2015-03-30 01:30", freq="D"
+        ).tz_localize(tz, nonexistent="shift_forward")
+        tm.assert_index_equal(result, expected)