fix: Method dt.offset_by was discaring month and year info if day w…

…as included in offset for timezone-aware columns

crates/polars-time/src/windows/duration.rs

@@ -870,7 +870,7 @@ impl Duration {
 
     fn add_impl_month_week_or_day<F, G, J>(
         &self,
-        t: i64,
+        mut t: i64,
         tz: Option<&Tz>,
         nsecs_to_unit: F,
         timestamp_to_datetime: G,
@@ -882,7 +882,6 @@ impl Duration {
         J: Fn(NaiveDateTime) -> i64,
     {
         let d = self;
-        let mut new_t = t;
 
         if d.months > 0 {
             let ts = match tz {
@@ -894,7 +893,7 @@ impl Duration {
                 _ => timestamp_to_datetime(t),
             };
             let dt = Self::add_month(ts, d.months, d.negative);
-            new_t = match tz {
+            t = match tz {
                 #[cfg(feature = "timezones")]
                 // for UTC, use fastpath below (same as naive)
                 Some(tz) if tz != &chrono_tz::UTC => datetime_to_timestamp(
@@ -911,20 +910,19 @@ impl Duration {
                 #[cfg(feature = "timezones")]
                 // for UTC, use fastpath below (same as naive)
                 Some(tz) if tz != &chrono_tz::UTC => {
-                    new_t =
-                        datetime_to_timestamp(unlocalize_datetime(timestamp_to_datetime(t), tz));
-                    new_t += if d.negative { -t_weeks } else { t_weeks };
-                    new_t = datetime_to_timestamp(
+                    t = datetime_to_timestamp(unlocalize_datetime(timestamp_to_datetime(t), tz));
+                    t += if d.negative { -t_weeks } else { t_weeks };
+                    t = datetime_to_timestamp(
                         try_localize_datetime(
-                            timestamp_to_datetime(new_t),
+                            timestamp_to_datetime(t),
                             tz,
                             Ambiguous::Raise,
                             NonExistent::Raise,
                         )?
                         .expect("we didn't use Ambiguous::Null or NonExistent::Null"),
                     );
                 },
-                _ => new_t += if d.negative { -t_weeks } else { t_weeks },
+                _ => t += if d.negative { -t_weeks } else { t_weeks },
             };
         }
 
@@ -934,24 +932,23 @@ impl Duration {
                 #[cfg(feature = "timezones")]
                 // for UTC, use fastpath below (same as naive)
                 Some(tz) if tz != &chrono_tz::UTC => {
-                    new_t =
-                        datetime_to_timestamp(unlocalize_datetime(timestamp_to_datetime(t), tz));
-                    new_t += if d.negative { -t_days } else { t_days };
-                    new_t = datetime_to_timestamp(
+                    t = datetime_to_timestamp(unlocalize_datetime(timestamp_to_datetime(t), tz));
+                    t += if d.negative { -t_days } else { t_days };
+                    t = datetime_to_timestamp(
                         try_localize_datetime(
-                            timestamp_to_datetime(new_t),
+                            timestamp_to_datetime(t),
                             tz,
                             Ambiguous::Raise,
                             NonExistent::Raise,
                         )?
                         .expect("we didn't use Ambiguous::Null or NonExistent::Null"),
                     );
                 },
-                _ => new_t += if d.negative { -t_days } else { t_days },
+                _ => t += if d.negative { -t_days } else { t_days },
             };
         }
 
-        Ok(new_t)
+        Ok(t)
     }
 
     pub fn add_ns(&self, t: i64, tz: Option<&Tz>) -> PolarsResult<i64> {

py-polars/tests/unit/operations/namespaces/temporal/test_offset_by.py

@@ -143,3 +143,24 @@ def test_offset_by_unique_29_feb_19608() -> None:
         schema_overrides={"t": pl.Datetime("ms", "UTC")},
     )
     assert_frame_equal(result, expected)
+
+
+def test_month_then_day_21283() -> None:
+    series_vienna = pl.Series(
+        [datetime(2024, 5, 15, 8, 0)], dtype=pl.Datetime(time_zone="Europe/Vienna")
+    )
+    result = series_vienna.dt.offset_by("2y1mo1q1h")[0]
+    expected = datetime.strptime("2026-09-15 11:00:00+02:00", "%Y-%m-%d %H:%M:%S%z")
+    assert result == expected
+    result = series_vienna.dt.offset_by("2y1mo1q1h1d")[0]
+    expected = datetime.strptime("2026-09-16 11:00:00+02:00", "%Y-%m-%d %H:%M:%S%z")
+    assert result == expected
+    series_utc = pl.Series(
+        [datetime(2024, 5, 15, 8, 0)], dtype=pl.Datetime(time_zone="UTC")
+    )
+    result = series_utc.dt.offset_by("2y1mo1q1h")[0]
+    expected = datetime.strptime("2026-09-15 09:00:00+00:00", "%Y-%m-%d %H:%M:%S%z")
+    assert result == expected
+    result = series_utc.dt.offset_by("2y1mo1q1h1d")[0]
+    expected = datetime.strptime("2026-09-16 09:00:00+00:00", "%Y-%m-%d %H:%M:%S%z")
+    assert result == expected