fix: avoid counting partial intervals as full months

rust/src/python/helpers.rs

@@ -341,14 +341,16 @@ pub fn precise_diff<'py>(
                     day_diff += days_in_last_month;
                 }
             }
-            Ordering::Equal => {
+            Ordering::Equal
+                if dtinfo1.day == days_in_last_month && dtinfo2.day == days_in_month =>
+            {
                 // We have exactly a full month
                 // We remove the days difference
                 // and add one to the months difference
                 day_diff = 0;
                 month_diff += 1;
             }
-            Ordering::Greater => {
+            Ordering::Equal | Ordering::Greater => {
                 // We have a full month
                 day_diff += days_in_last_month;
             }

src/pendulum/_helpers.py

@@ -278,7 +278,11 @@ def precise_diff(
                 d_diff += d1.day
             else:
                 d_diff += days_in_last_month
-        elif d_diff == days_in_month - days_in_last_month:
+        elif (
+            d_diff == days_in_month - days_in_last_month
+            and d1.day == days_in_last_month
+            and d2.day == days_in_month
+        ):
             # We have exactly a full month
             # We remove the days difference
             # and add one to the months difference

tests/date/test_diff.py

@@ -261,7 +261,7 @@ def test_diff_for_humans_other_and_month():
 
         assert today.diff_for_humans(today.add(months=1)) == "1 month before"
 
-    with pendulum.travel_to(pendulum.datetime(2017, 1, 31)):
+    with pendulum.travel_to(pendulum.datetime(2017, 1, 31, tz="America/Toronto")):
         today = pendulum.today().date()
 
         assert today.diff_for_humans(today.add(weeks=4)) == "1 month before"
@@ -314,7 +314,7 @@ def test_diff_for_humans_other_and_future_month():
         assert today.diff_for_humans(today.subtract(weeks=4)) == "4 weeks after"
         assert today.diff_for_humans(today.subtract(months=1)) == "1 month after"
 
-    with pendulum.travel_to(pendulum.datetime(2017, 2, 28)):
+    with pendulum.travel_to(pendulum.datetime(2017, 2, 28, tz="America/Toronto")):
         today = pendulum.today().date()
 
         assert today.diff_for_humans(today.subtract(weeks=4)) == "1 month after"

tests/test_helpers.py

@@ -126,6 +126,44 @@ def test_precise_diff_timezone() -> None:
     assert diff.total_days == 0
 
 
+@pytest.mark.parametrize(
+    "year, start_day, expected_days", [(2023, 4, 28), (2024, 3, 29)]
+)
+@pytest.mark.parametrize("reverse", [False, True])
+def test_precise_diff_partial_month(
+    year: int, start_day: int, expected_days: int, reverse: bool
+) -> None:
+    start = datetime(year, 1, start_day)
+    end = datetime(year, 2, 1)
+    sign = -1 if reverse else 1
+    diff = precise_diff(end, start) if reverse else precise_diff(start, end)
+
+    assert_diff(diff, days=sign * expected_days)
+
+    start = pendulum.datetime(year, 1, start_day, tz="America/Toronto")
+    end = pendulum.datetime(year, 2, 1, tz="America/Toronto")
+    interval = start - end if reverse else end - start
+
+    assert interval.in_months() == 0
+    assert interval.in_days() == sign * expected_days
+
+
+@pytest.mark.parametrize("year, end_day", [(2023, 28), (2024, 29)])
+def test_precise_diff_clipped_full_month(year: int, end_day: int) -> None:
+    start = datetime(year, 1, 31)
+    end = datetime(year, 2, end_day)
+
+    assert_diff(precise_diff(start, end), months=1)
+    assert_diff(precise_diff(end, start), months=-1)
+
+
+def test_precise_diff_partial_month_with_borrowed_day() -> None:
+    start = datetime(2024, 1, 30, 1)
+    end = datetime(2024, 2, 29)
+
+    assert_diff(precise_diff(start, end), days=29, hours=23)
+
+
 def test_week_day() -> None:
     assert week_day(2017, 6, 2) == 5
     assert week_day(2017, 1, 1) == 7