Preserve calendar units in reflected Date addition

src/pendulum/date.py

@@ -260,6 +260,9 @@ def __add__(self, other: timedelta) -> Self:
 
         return self._add_timedelta(other)
 
+    def __radd__(self, other: timedelta) -> Self:
+        return self.__add__(other)
+
     @overload  # type: ignore[override]  # this is only needed because of Python 3.7
     def __sub__(self, __delta: timedelta) -> Self: ...
 

tests/date/test_add.py

@@ -78,6 +78,53 @@ def test_add_duration():
     assert_date(new, 2017, 7, 2)
 
 
+@pytest.mark.parametrize(
+    "date, duration, expected",
+    [
+        ((2024, 1, 31), {"months": 1}, (2024, 2, 29)),
+        ((2023, 1, 31), {"months": 1}, (2023, 2, 28)),
+        ((2024, 3, 31), {"months": -1}, (2024, 2, 29)),
+        ((2023, 3, 31), {"months": -1}, (2023, 2, 28)),
+        ((2023, 3, 1), {"years": 1}, (2024, 3, 1)),
+        ((2024, 2, 29), {"years": -1}, (2023, 2, 28)),
+        ((2023, 1, 31), {"years": 2, "months": 3, "days": 18}, (2025, 5, 18)),
+    ],
+)
+def test_add_duration_reflected(date, duration, expected):
+    d = pendulum.Date(*date)
+    delta = pendulum.duration(**duration)
+
+    result = delta + d
+
+    assert isinstance(result, pendulum.Date)
+    assert_date(result, *expected)
+    assert result == d + delta
+
+
+@pytest.mark.parametrize(
+    "delta",
+    [timedelta(days=18), timedelta(seconds=-1), timedelta(), timedelta(seconds=1)],
+)
+def test_add_timedelta_reflected(delta):
+    d = pendulum.Date(2024, 1, 31)
+
+    result = delta + d
+
+    assert isinstance(result, pendulum.Date)
+    assert result == d + delta
+
+
+def test_add_duration_reflected_preserves_date_subclass():
+    class CustomDate(pendulum.Date):
+        pass
+
+    d = CustomDate(2024, 1, 31)
+    result = pendulum.duration(months=1) + d
+
+    assert isinstance(result, CustomDate)
+    assert_date(result, 2024, 2, 29)
+
+
 def test_addition_invalid_type():
     d = pendulum.date(2015, 3, 14)