BUG: Decrement start date with a negative, non-Tick frequency (#23278)

Author: mroeschke

doc/source/whatsnew/v0.24.0.txt

@@ -996,6 +996,7 @@ Datetimelike
 - Bug in rounding methods of :class:`DatetimeIndex` (:meth:`~DatetimeIndex.round`, :meth:`~DatetimeIndex.ceil`, :meth:`~DatetimeIndex.floor`) and :class:`Timestamp` (:meth:`~Timestamp.round`, :meth:`~Timestamp.ceil`, :meth:`~Timestamp.floor`) could give rise to loss of precision (:issue:`22591`)
 - Bug in :func:`to_datetime` with an :class:`Index` argument that would drop the ``name`` from the result (:issue:`21697`)
 - Bug in :class:`PeriodIndex` where adding or subtracting a :class:`timedelta` or :class:`Tick` object produced incorrect results (:issue:`22988`)
+- Bug in :func:`date_range` when decrementing a start date to a past end date by a negative frequency (:issue:`23270`)
 
 Timedelta
 ^^^^^^^^^

pandas/tests/indexes/datetimes/test_date_range.py

@@ -510,6 +510,16 @@ def test_timezone_comparaison_assert(self):
         with tm.assert_raises_regex(AssertionError, msg):
             date_range(start, periods=2, tz='Europe/Berlin')
 
+    def test_negative_non_tick_frequency_descending_dates(self,
+                                                          tz_aware_fixture):
+        # GH 23270
+        tz = tz_aware_fixture
+        result = pd.date_range(start='2011-06-01', end='2011-01-01',
+                               freq='-1MS', tz=tz)
+        expected = pd.date_range(end='2011-06-01', start='2011-01-01',
+                                 freq='1MS', tz=tz)[::-1]
+        tm.assert_index_equal(result, expected)
+
 
 class TestGenRangeGeneration(object):
 

pandas/tseries/offsets.py

@@ -2388,7 +2388,7 @@ def generate_range(start=None, end=None, periods=None,
     elif end and not offset.onOffset(end):
         end = offset.rollback(end)
 
-    if periods is None and end < start:
+    if periods is None and end < start and offset.n >= 0:
         end = None
         periods = 0