fix: Sortedness was incorrectly being preserved in dt.offset_by when offsetting by non-constant durations in the timezone-naive case

crates/polars-time/src/offset_by.rs

@@ -58,20 +58,35 @@ fn apply_offsets_to_datetime(
 }
 
 pub fn impl_offset_by(ts: &Series, offsets: &Series) -> PolarsResult<Series> {
-    let preserve_sortedness: bool;
     let offsets = offsets.str()?;
-    let out = match ts.dtype() {
+    let dtype = ts.dtype();
+
+    // Sortedness may not be preserved for non-constant durations,
+    // see https://github.com/pola-rs/polars/issues/19608 for a counterexample.
+    // Constant durations (e.g. 2 hours) always preserve sortedness.
+    let tz = match dtype {
+        DataType::Date => None,
+        DataType::Datetime(_, tz) => tz.clone(),
+        _ => polars_bail!(InvalidOperation: "expected Date or Datetime, got {}", dtype),
+    };
+    let preserve_sortedness = match offsets.len() {
+        1 => match offsets.get(0) {
+            Some(offset) => {
+                let offset = Duration::parse(offset);
+                offset.is_constant_duration(tz.as_deref())
+            },
+            None => false,
+        },
+        _ => false,
+    };
+
+    let out = match dtype {
         DataType::Date => {
             let ts = ts
                 .cast(&DataType::Datetime(TimeUnit::Milliseconds, None))
                 .unwrap();
             let datetime = ts.datetime().unwrap();
             let out = apply_offsets_to_datetime(datetime, offsets, None)?;
-            // sortedness is only guaranteed to be preserved if a constant offset is being added to every datetime
-            preserve_sortedness = match offsets.len() {
-                1 => offsets.get(0).is_some(),
-                _ => false,
-            };
             out.cast(&DataType::Datetime(TimeUnit::Milliseconds, None))
                 .unwrap()
                 .cast(&DataType::Date)
@@ -86,21 +101,6 @@ pub fn impl_offset_by(ts: &Series, offsets: &Series) -> PolarsResult<Series> {
                 },
                 _ => apply_offsets_to_datetime(datetime, offsets, None)?,
             };
-            // Sortedness may not be preserved when crossing daylight savings time boundaries
-            // for calendar-aware durations.
-            // Constant durations (e.g. 2 hours) always preserve sortedness.
-            preserve_sortedness = match offsets.len() {
-                1 => match offsets.get(0) {
-                    Some(offset) => {
-                        let offset = Duration::parse(offset);
-                        tz.is_none()
-                            || tz.as_deref() == Some("UTC")
-                            || offset.is_constant_duration(tz.as_deref())
-                    },
-                    None => false,
-                },
-                _ => false,
-            };
             out.cast(&DataType::Datetime(*tu, tz.clone()))
         },
         dt => polars_bail!(

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

@@ -178,9 +178,9 @@ def test_local_time_before_epoch(time_unit: TimeUnit) -> None:
         (None, "1d", True),
         ("Europe/London", "1d", False),
         ("UTC", "1d", True),
-        (None, "1mo", True),
-        ("Europe/London", "1mo", False),
-        ("UTC", "1mo", True),
+        (None, "1m", True),
+        ("Europe/London", "1m", True),
+        ("UTC", "1m", True),
         (None, "1w", True),
         ("Europe/London", "1w", False),
         ("UTC", "1w", True),

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

@@ -6,7 +6,7 @@
 import pytest
 
 import polars as pl
-from polars.testing import assert_series_equal
+from polars.testing import assert_frame_equal, assert_series_equal
 
 if TYPE_CHECKING:
     from polars._typing import TimeUnit
@@ -116,3 +116,30 @@ def test_datetime_offset_by(
         time_zone
     )
     assert_series_equal(result, expected)
+
+
+def test_offset_by_unique_29_feb_19608() -> None:
+    df20 = pl.select(
+        t=pl.datetime_range(
+            pl.datetime(2020, 2, 28),
+            pl.datetime(2020, 3, 1),
+            closed="left",
+            time_unit="ms",
+            interval="8h",
+            time_zone="UTC",
+        ),
+    ).with_columns(x=pl.int_range(pl.len()))
+    df19 = df20.with_columns(pl.col("t").dt.offset_by("-1y"))
+    result = df19.unique("t", keep="first").sort("t")
+    expected = pl.DataFrame(
+        {
+            "t": [
+                datetime(2019, 2, 28),
+                datetime(2019, 2, 28, 8),
+                datetime(2019, 2, 28, 16),
+            ],
+            "x": [0, 1, 2],
+        },
+        schema_overrides={"t": pl.Datetime("ms", "UTC")},
+    )
+    assert_frame_equal(result, expected)