From 8882e94220ae6b5d729fd8263cd68a408ae5ff79 Mon Sep 17 00:00:00 2001
From: fan wu <336798967+FanWu-ai@users.noreply.github.com>
Date: Wed, 21 Oct 2026 06:33:50 +0900
Subject: [PATCH] Include dates inside inverted intervals

---
 docs/docs/interval.md                        |  3 +
 src/pendulum/interval.py                     |  5 +-
 tests/interval/test_membership_regression.py | 61 ++++++++++++++++++++
 3 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 tests/interval/test_membership_regression.py

diff --git a/docs/docs/interval.md b/docs/docs/interval.md
index fc70fb3c4..1ff4347ce 100644
--- a/docs/docs/interval.md
+++ b/docs/docs/interval.md
@@ -166,3 +166,6 @@ You can check if a `DateTime` instance is inside a interval using the `in` keywo
 >>> dt in interval
 True
 ```
+
+Membership includes both endpoints, including for inverted intervals. The order of
+the endpoints controls iteration direction, not whether a date is inside the interval.
diff --git a/src/pendulum/interval.py b/src/pendulum/interval.py
index c5e0713a7..5574e8927 100644
--- a/src/pendulum/interval.py
+++ b/src/pendulum/interval.py
@@ -314,7 +314,10 @@ def __iter__(self) -> Iterator[_T]:
         return self.range("days")
 
     def __contains__(self, item: _T) -> bool:
-        return self.start <= item <= self.end
+        if self.start <= self.end:
+            return self.start <= item <= self.end
+
+        return self.end <= item <= self.start
 
     def __add__(self, other: timedelta) -> Duration:  # type: ignore[override]
         return self.as_duration().__add__(other)
diff --git a/tests/interval/test_membership_regression.py b/tests/interval/test_membership_regression.py
new file mode 100644
index 000000000..2d8d0987f
--- /dev/null
+++ b/tests/interval/test_membership_regression.py
@@ -0,0 +1,61 @@
+from __future__ import annotations
+
+from datetime import date
+from datetime import datetime
+from datetime import timezone
+from functools import partial
+from typing import TYPE_CHECKING
+
+import pytest
+
+import pendulum
+
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
+
+
+@pytest.mark.parametrize(
+    "factory",
+    [date, partial(datetime, tzinfo=timezone.utc), pendulum.date, pendulum.datetime],
+)
+@pytest.mark.parametrize("reverse", [False, True])
+@pytest.mark.parametrize("absolute", [False, True])
+def test_interval_membership_includes_both_endpoints(
+    factory: Callable[[int, int, int], date], reverse: bool, absolute: bool
+) -> None:
+    first = factory(2024, 1, 10)
+    last = factory(2024, 1, 12)
+    start, end = (last, first) if reverse else (first, last)
+    interval = pendulum.Interval(start, end, absolute=absolute)
+
+    for day in (10, 11, 12):
+        assert factory(2024, 1, day) in interval
+    for day in (9, 13):
+        assert factory(2024, 1, day) not in interval
+
+    assert interval.start == (first if absolute else start)
+    assert interval.end == (last if absolute else end)
+
+
+@pytest.mark.parametrize(
+    "factory",
+    [date, partial(datetime, tzinfo=timezone.utc), pendulum.date, pendulum.datetime],
+)
+def test_single_point_interval_membership(
+    factory: Callable[[int, int, int], date],
+) -> None:
+    point = factory(2024, 1, 10)
+    interval = pendulum.Interval(point, point)
+    assert point in interval
+    assert factory(2024, 1, 9) not in interval
+    assert factory(2024, 1, 11) not in interval
+
+
+def test_inverted_interval_membership_with_different_offsets() -> None:
+    start = pendulum.datetime(2024, 1, 10, 16, tz="Europe/Paris")
+    end = pendulum.datetime(2024, 1, 10, 9, tz="America/New_York")
+    interval = pendulum.Interval(start, end)
+    assert pendulum.datetime(2024, 1, 10, 14, 30, tz="UTC") in interval
+    assert pendulum.datetime(2024, 1, 10, 13, 59, tz="UTC") not in interval
+    assert pendulum.datetime(2024, 1, 10, 15, 1, tz="UTC") not in interval