Keep years and months when adding or subtracting durations.

Rebuilding the sum from total seconds folds calendar fields into a 365/30-day approximation, so two year-long durations no longer add up to two calendar years.

Author: fhgffy

src/pendulum/duration.py

@@ -329,8 +329,34 @@ def __repr__(self) -> str:
 
         return rep.replace(", )", ")")
 
+    def _scalar_seconds(self) -> float:
+        """Seconds from weeks, days, and the clock, excluding years and months.
+
+        ``Duration`` stores years and months beside a ``timedelta`` that also
+        contains their 365/30-day approximation. Adding those totals together
+        and rebuilding from seconds alone drops the calendar fields (#751).
+        """
+        calendar = (self._years * 365 + self._months * 30) * SECONDS_PER_DAY
+        return self.total_seconds() - calendar
+
     def __add__(self, other: timedelta) -> Self:
         if isinstance(other, timedelta):
+            if (
+                type(self) is Duration
+                and isinstance(other, Duration)
+                and type(other) is Duration
+            ):
+                return self.__class__(
+                    years=self._years + other._years,
+                    months=self._months + other._months,
+                    seconds=self._scalar_seconds() + other._scalar_seconds(),
+                )
+            if type(self) is Duration and (self._years or self._months):
+                return self.__class__(
+                    years=self._years,
+                    months=self._months,
+                    seconds=self._scalar_seconds() + other.total_seconds(),
+                )
             return self.__class__(seconds=self.total_seconds() + other.total_seconds())
 
         return NotImplemented
@@ -339,6 +365,22 @@ def __add__(self, other: timedelta) -> Self:
 
     def __sub__(self, other: timedelta) -> Self:
         if isinstance(other, timedelta):
+            if (
+                type(self) is Duration
+                and isinstance(other, Duration)
+                and type(other) is Duration
+            ):
+                return self.__class__(
+                    years=self._years - other._years,
+                    months=self._months - other._months,
+                    seconds=self._scalar_seconds() - other._scalar_seconds(),
+                )
+            if type(self) is Duration and (self._years or self._months):
+                return self.__class__(
+                    years=self._years,
+                    months=self._months,
+                    seconds=self._scalar_seconds() - other.total_seconds(),
+                )
             return self.__class__(seconds=self.total_seconds() - other.total_seconds())
 
         return NotImplemented

tests/duration/test_add_sub.py

@@ -49,6 +49,32 @@ def test_sub_unsupported():
     assert NotImplemented == p.__sub__(5)
 
 
+def test_add_preserves_years_and_months():
+    total = pendulum.duration(years=2) + pendulum.duration(days=1)
+    assert_duration(total, years=2, months=0, weeks=0, days=1)
+
+    months = pendulum.duration(months=2) + pendulum.duration(days=1)
+    assert_duration(months, years=0, months=2, days=1)
+
+    combined = pendulum.duration(years=1) + pendulum.duration(years=1)
+    assert combined.years == 2
+    assert combined.months == 0
+
+
+def test_adding_year_durations_matches_successive_datetime_adds():
+    start = pendulum.datetime(2023, 1, 1, tz="UTC")
+    stepwise = (start + pendulum.duration(years=1)) + pendulum.duration(years=1)
+    combined = start + (pendulum.duration(years=1) + pendulum.duration(years=1))
+
+    assert stepwise == combined
+    assert combined == pendulum.datetime(2025, 1, 1, tz="UTC")
+
+
+def test_sub_preserves_years():
+    difference = pendulum.duration(years=2, days=3) - pendulum.duration(years=1, days=1)
+    assert_duration(difference, years=1, days=2)
+
+
 def test_neg():
     p = pendulum.duration(days=23, seconds=32)
     assert_duration(-p, 0, 0, -3, -2, 0, 0, -32)