Recognize dates inside reversed intervals

src/pendulum/interval.py

@@ -314,6 +314,8 @@ def __iter__(self) -> Iterator[_T]:
         return self.range("days")
 
     def __contains__(self, item: _T) -> bool:
+        if self.start > self.end:
+            return self.end <= item <= self.start
         return self.start <= item <= self.end
 
     def __add__(self, other: timedelta) -> Duration:  # type: ignore[override]

tests/interval/test_reversed_membership.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+import pytest
+
+import pendulum
+
+
+@pytest.mark.parametrize("use_datetime", [False, True])
+@pytest.mark.parametrize("reverse", [False, True])
+@pytest.mark.parametrize("day", [1, 2, 3, 4, 5])
+def test_membership_matches_interval_bounds(
+    use_datetime: bool, reverse: bool, day: int
+) -> None:
+    start: pendulum.Date
+    end: pendulum.Date
+    item: pendulum.Date
+    if use_datetime:
+        start = pendulum.datetime(2026, 1, 2)
+        end = pendulum.datetime(2026, 1, 4)
+        item = pendulum.datetime(2026, 1, day)
+    else:
+        start = pendulum.date(2026, 1, 2)
+        end = pendulum.date(2026, 1, 4)
+        item = pendulum.date(2026, 1, day)
+    interval = (
+        pendulum.Interval(end, start) if reverse else pendulum.Interval(start, end)
+    )
+    assert (item in interval) == (2 <= day <= 4)