BUG: pydatetime + Timedelta[non-nano] incorrect results

doc/source/whatsnew/v3.0.0.rst

@@ -1131,6 +1131,7 @@ Timedelta
 - Accuracy improvement in :meth:`Timedelta.to_pytimedelta` to round microseconds consistently for large nanosecond based Timedelta (:issue:`57841`)
 - Bug in :class:`Timedelta` constructor failing to raise when passed an invalid keyword (:issue:`53801`)
 - Bug in :meth:`DataFrame.cumsum` which was raising ``IndexError`` if dtype is ``timedelta64[ns]`` (:issue:`57956`)
+- Bug in adding or subtracting a :class:`Timedelta` object with non-nanosecond unit to a python ``datetime.datetime`` object giving incorrect results; this now works correctly for Timedeltas inside the ``datetime.timedelta`` implementation bounds (:issue:`53643`)
 - Bug in multiplication operations with ``timedelta64`` dtype failing to raise ``TypeError`` when multiplying by ``bool`` objects or dtypes (:issue:`58054`)
 - Bug in multiplication operations with ``timedelta64`` dtype incorrectly raising when multiplying by numpy-nullable dtypes or pyarrow integer dtypes (:issue:`58054`)
 

pandas/_libs/tslibs/timedeltas.pyx

@@ -1022,9 +1022,23 @@ cdef _timedelta_from_value_and_reso(cls, int64_t value, NPY_DATETIMEUNIT reso):
     elif reso == NPY_DATETIMEUNIT.NPY_FR_us:
         td_base = _Timedelta.__new__(cls, microseconds=int(value))
     elif reso == NPY_DATETIMEUNIT.NPY_FR_ms:
-        td_base = _Timedelta.__new__(cls, milliseconds=0)
+        if -86_399_999_913_600_000 <= value <= 86_400_000_000_000_000:
+            # i.e. we are in range for pytimedelta. By passing the
+            #  'correct' value here we can
+            #   make pydatetime + Timedelta operations work correctly,
+            #  xref GH#53643
+            td_base = _Timedelta.__new__(cls, milliseconds=value)
+        else:
+            td_base = _Timedelta.__new__(cls, milliseconds=0)
     elif reso == NPY_DATETIMEUNIT.NPY_FR_s:
-        td_base = _Timedelta.__new__(cls, seconds=0)
+        if -86_399_999_913_600 <= value <= 86_400_000_000_000:
+            # i.e. we are in range for pytimedelta. By passing the
+            #  'correct' value here we can
+            #   make pydatetime + Timedelta operations work correctly,
+            #  xref GH#53643
+            td_base = _Timedelta.__new__(cls, seconds=value)
+        else:
+            td_base = _Timedelta.__new__(cls, seconds=0)
     # Other resolutions are disabled but could potentially be implemented here:
     # elif reso == NPY_DATETIMEUNIT.NPY_FR_m:
     #    td_base = _Timedelta.__new__(Timedelta, minutes=int(value))

pandas/tests/scalar/timedelta/test_arithmetic.py

@@ -35,6 +35,21 @@ class TestTimedeltaAdditionSubtraction:
         __sub__, __rsub__
     """
 
+    def test_td_add_sub_pydatetime(self, unit):
+        # GH#53643
+        td = Timedelta(hours=23).as_unit(unit)
+        dt = datetime(2016, 1, 1)
+
+        expected = datetime(2016, 1, 1, 23)
+        result = dt + td
+        assert result == expected
+        result = td + dt
+        assert result == expected
+
+        expected = datetime(2015, 12, 31, 1)
+        result = dt - td
+        assert result == expected
+
     @pytest.mark.parametrize(
         "ten_seconds",
         [