From 584d68020eeb8946d2f25188b72802be2836c15b Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 15 Jul 2024 10:14:26 +0100
Subject: [PATCH 1/6] feat: implement round and truncate for Duration

---
 .../src/dsl/function_expr/datetime.rs         |  14 +-
 crates/polars-time/src/round.rs               |  83 +++++++--
 crates/polars-time/src/truncate.rs            |  77 ++++++--
 py-polars/polars/expr/datetime.py             |  21 ++-
 py-polars/polars/series/datetime.py           |  27 ++-
 .../namespaces/temporal/test_datetime.py      | 173 ++++++++++++++++--
 6 files changed, 325 insertions(+), 70 deletions(-)

diff --git a/crates/polars-plan/src/dsl/function_expr/datetime.rs b/crates/polars-plan/src/dsl/function_expr/datetime.rs
index 1a6251a43f82..ea1c6f17c225 100644
--- a/crates/polars-plan/src/dsl/function_expr/datetime.rs
+++ b/crates/polars-plan/src/dsl/function_expr/datetime.rs
@@ -397,7 +397,12 @@ pub(super) fn truncate(s: &[Series]) -> PolarsResult<Series> {
             _ => time_series.datetime()?.truncate(None, every)?.into_series(),
         },
         DataType::Date => time_series.date()?.truncate(None, every)?.into_series(),
-        dt => polars_bail!(opq = round, got = dt, expected = "date/datetime"),
+        DataType::Duration(_) => time_series.duration()?.truncate(None, every)?.into_series(),
+        dt => polars_bail!(
+            opq = truncate,
+            got = dt,
+            expected = "date/datetime/duration"
+        ),
     };
     out.set_sorted_flag(time_series.is_sorted_flag());
     Ok(out)
@@ -498,7 +503,12 @@ pub(super) fn round(s: &[Series]) -> PolarsResult<Series> {
             .unwrap()
             .round(every, None)?
             .into_series(),
-        dt => polars_bail!(opq = round, got = dt, expected = "date/datetime"),
+        DataType::Duration(_) => time_series
+            .duration()
+            .unwrap()
+            .round(every, None)?
+            .into_series(),
+        dt => polars_bail!(opq = round, got = dt, expected = "date/datetime/duration"),
     })
 }
 
diff --git a/crates/polars-time/src/round.rs b/crates/polars-time/src/round.rs
index 4bb6f2a3386f..ab0878c3d40d 100644
--- a/crates/polars-time/src/round.rs
+++ b/crates/polars-time/src/round.rs
@@ -21,9 +21,7 @@ impl PolarsRound for DatetimeChunked {
         if every.len() == 1 {
             if let Some(every) = every.get(0) {
                 let every_parsed = Duration::parse(every);
-                if every_parsed.negative {
-                    polars_bail!(ComputeError: "cannot round a Datetime to a negative duration")
-                }
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Datetime to a non-positive duration");
                 if (time_zone.is_none() || time_zone.as_deref() == Some("UTC"))
                     && (every_parsed.months() == 0 && every_parsed.weeks() == 0)
                 {
@@ -76,14 +74,11 @@ impl PolarsRound for DatetimeChunked {
             opt_every,
         ) {
             (Some(timestamp), Some(every)) => {
-                let every =
+                let every_parsed =
                     *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Date to a non-positive duration");
 
-                if every.negative {
-                    polars_bail!(ComputeError: "cannot round a Datetime to a negative duration")
-                }
-
-                let w = Window::new(every, every, offset);
+                let w = Window::new(every_parsed, every_parsed, offset);
                 func(&w, timestamp, tz).map(Some)
             },
             _ => Ok(None),
@@ -98,11 +93,9 @@ impl PolarsRound for DateChunked {
         let out = match every.len() {
             1 => {
                 if let Some(every) = every.get(0) {
-                    let every = Duration::parse(every);
-                    if every.negative {
-                        polars_bail!(ComputeError: "cannot round a Date to a negative duration")
-                    }
-                    let w = Window::new(every, every, offset);
+                    let every_parsed = Duration::parse(every);
+                    polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Date to a non-positive duration");
+                    let w = Window::new(every_parsed, every_parsed, offset);
                     self.try_apply_nonnull_values_generic(|t| {
                         Ok(
                             (w.round_ms(MILLISECONDS_IN_DAY * t as i64, None)?
@@ -118,14 +111,11 @@ impl PolarsRound for DateChunked {
                 let mut duration_cache = FastFixedCache::new((every.len() as f64).sqrt() as usize);
                 match (opt_t, opt_every) {
                     (Some(t), Some(every)) => {
-                        let every = *duration_cache
+                        let every_parsed = *duration_cache
                             .get_or_insert_with(every, |every| Duration::parse(every));
+                        polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Date to a non-positive duration");
 
-                        if every.negative {
-                            polars_bail!(ComputeError: "cannot round a Date to a negative duration")
-                        }
-
-                        let w = Window::new(every, every, offset);
+                        let w = Window::new(every_parsed, every_parsed, offset);
                         Ok(Some(
                             (w.round_ms(MILLISECONDS_IN_DAY * t as i64, None)?
                                 / MILLISECONDS_IN_DAY) as i32,
@@ -138,3 +128,56 @@ impl PolarsRound for DateChunked {
         Ok(out?.into_date())
     }
 }
+
+#[cfg(feature = "dtype-duration")]
+impl PolarsRound for DurationChunked {
+    fn round(&self, every: &StringChunked, _tz: Option<&Tz>) -> PolarsResult<Self> {
+        if every.len() == 1 {
+            if let Some(every) = every.get(0) {
+                let every_parsed = Duration::parse(every);
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Duration to a non-positive duration");
+                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot round a Duration to a non-constant duration (i.e. one that involves weeks / months)");
+                let every = match self.time_unit() {
+                    TimeUnit::Milliseconds => every_parsed.duration_ms(),
+                    TimeUnit::Microseconds => every_parsed.duration_us(),
+                    TimeUnit::Nanoseconds => every_parsed.duration_ns(),
+                };
+                return Ok(self
+                    .apply_values(|t| {
+                        // Round half-way values away from zero
+                        let half_away = t.signum() * every / 2;
+                        t + half_away - (t + half_away) % every
+                    })
+                    .into_duration(self.time_unit()));
+            } else {
+                return Ok(Int64Chunked::full_null(self.name(), self.len())
+                    .into_duration(self.time_unit()));
+            }
+        }
+
+        // A sqrt(n) cache is not too small, not too large.
+        let mut duration_cache = FastFixedCache::new((every.len() as f64).sqrt() as usize);
+
+        let out = broadcast_try_binary_elementwise(self, every, |opt_timestamp, opt_every| match (
+            opt_timestamp,
+            opt_every,
+        ) {
+            (Some(t), Some(every)) => {
+                let every_parsed =
+                    *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
+                polars_ensure!(!every_parsed.negative, InvalidOperation: "cannot round a Duration to a negative duration");
+                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot round a Duration to a non-constant duration (i.e. one that involves weeks / months)");
+                let every = match self.time_unit() {
+                    TimeUnit::Milliseconds => every_parsed.duration_ms(),
+                    TimeUnit::Microseconds => every_parsed.duration_us(),
+                    TimeUnit::Nanoseconds => every_parsed.duration_ns(),
+                };
+                // Round half-way values away from zero
+                let half_away = t.signum() * every / 2;
+                Ok(Some(t + half_away - (t + half_away) % every))
+            },
+            _ => Ok(None),
+        });
+        Ok(out?.into_duration(self.time_unit()))
+    }
+}
diff --git a/crates/polars-time/src/truncate.rs b/crates/polars-time/src/truncate.rs
index 991ce50b547a..a4cf11ab47e1 100644
--- a/crates/polars-time/src/truncate.rs
+++ b/crates/polars-time/src/truncate.rs
@@ -21,9 +21,7 @@ impl PolarsTruncate for DatetimeChunked {
         if every.len() == 1 {
             if let Some(every) = every.get(0) {
                 let every_parsed = Duration::parse(every);
-                if every_parsed.negative {
-                    polars_bail!(ComputeError: "cannot truncate a Datetime to a negative duration")
-                }
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Datetime to a non-positive duration");
                 if (time_zone.is_none() || time_zone.as_deref() == Some("UTC"))
                     && (every_parsed.months() == 0 && every_parsed.weeks() == 0)
                 {
@@ -75,14 +73,11 @@ impl PolarsTruncate for DatetimeChunked {
             opt_every,
         ) {
             (Some(timestamp), Some(every)) => {
-                let every =
+                let every_parsed =
                     *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Datetime to a non-positive duration");
 
-                if every.negative {
-                    polars_bail!(ComputeError: "cannot truncate a Datetime to a negative duration")
-                }
-
-                let w = Window::new(every, every, offset);
+                let w = Window::new(every_parsed, every_parsed, offset);
                 func(&w, timestamp, tz).map(Some)
             },
             _ => Ok(None),
@@ -97,11 +92,9 @@ impl PolarsTruncate for DateChunked {
         let out = match every.len() {
             1 => {
                 if let Some(every) = every.get(0) {
-                    let every = Duration::parse(every);
-                    if every.negative {
-                        polars_bail!(ComputeError: "cannot truncate a Date to a negative duration")
-                    }
-                    let w = Window::new(every, every, offset);
+                    let every_parsed = Duration::parse(every);
+                    polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Date to a non-positive duration");
+                    let w = Window::new(every_parsed, every_parsed, offset);
                     self.try_apply_nonnull_values_generic(|t| {
                         Ok((w.truncate_ms(MILLISECONDS_IN_DAY * t as i64, None)?
                             / MILLISECONDS_IN_DAY) as i32)
@@ -115,14 +108,11 @@ impl PolarsTruncate for DateChunked {
                 let mut duration_cache = FastFixedCache::new((every.len() as f64).sqrt() as usize);
                 match (opt_t, opt_every) {
                     (Some(t), Some(every)) => {
-                        let every = *duration_cache
+                        let every_parsed = *duration_cache
                             .get_or_insert_with(every, |every| Duration::parse(every));
+                        polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Date to a non-positive duration");
 
-                        if every.negative {
-                            polars_bail!(ComputeError: "cannot truncate a Date to a negative duration")
-                        }
-
-                        let w = Window::new(every, every, offset);
+                        let w = Window::new(every_parsed, every_parsed, offset);
                         Ok(Some(
                             (w.truncate_ms(MILLISECONDS_IN_DAY * t as i64, None)?
                                 / MILLISECONDS_IN_DAY) as i32,
@@ -135,3 +125,50 @@ impl PolarsTruncate for DateChunked {
         Ok(out?.into_date())
     }
 }
+
+#[cfg(feature = "dtype-duration")]
+impl PolarsTruncate for DurationChunked {
+    fn truncate(&self, _tz: Option<&Tz>, every: &StringChunked) -> PolarsResult<Self> {
+        if every.len() == 1 {
+            if let Some(every) = every.get(0) {
+                let every_parsed = Duration::parse(every);
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Duration to a non-positive duration");
+                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot truncate a Duration to a non-constant duration (i.e. one that involves weeks / months)");
+                let every = match self.time_unit() {
+                    TimeUnit::Milliseconds => every_parsed.duration_ms(),
+                    TimeUnit::Microseconds => every_parsed.duration_us(),
+                    TimeUnit::Nanoseconds => every_parsed.duration_ns(),
+                };
+                return Ok(self
+                    .apply_values(|t| t - t % every)
+                    .into_duration(self.time_unit()));
+            } else {
+                return Ok(Int64Chunked::full_null(self.name(), self.len())
+                    .into_duration(self.time_unit()));
+            }
+        }
+
+        // A sqrt(n) cache is not too small, not too large.
+        let mut duration_cache = FastFixedCache::new((every.len() as f64).sqrt() as usize);
+
+        let out = broadcast_try_binary_elementwise(self, every, |opt_timestamp, opt_every| match (
+            opt_timestamp,
+            opt_every,
+        ) {
+            (Some(t), Some(every)) => {
+                let every_parsed =
+                    *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Duration to a non-positive duration");
+                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot truncate a Duration to a non-constant duration (i.e. one that involves weeks / months)");
+                let every = match self.time_unit() {
+                    TimeUnit::Milliseconds => every_parsed.duration_ms(),
+                    TimeUnit::Microseconds => every_parsed.duration_us(),
+                    TimeUnit::Nanoseconds => every_parsed.duration_ns(),
+                };
+                Ok(Some(t - t % every))
+            },
+            _ => Ok(None),
+        });
+        Ok(out?.into_duration(self.time_unit()))
+    }
+}
diff --git a/py-polars/polars/expr/datetime.py b/py-polars/polars/expr/datetime.py
index cdf6ccb6516f..528b61984b92 100644
--- a/py-polars/polars/expr/datetime.py
+++ b/py-polars/polars/expr/datetime.py
@@ -152,10 +152,11 @@ def add_business_days(
 
     def truncate(self, every: str | dt.timedelta | Expr) -> Expr:
         """
-        Divide the date/datetime range into buckets.
+        Divide the dates, datetimes, or durations into buckets.
 
-        Each date/datetime is mapped to the start of its bucket using the corresponding
-        local datetime. Note that weekly buckets start on Monday.
+        For dates or datetimes, each date/datetime is mapped to the start of its bucket
+        using the corresponding local datetime.
+        Note that weekly buckets start on Monday.
         Ambiguous results are localised using the DST offset of the original timestamp -
         for example, truncating `'2022-11-06 01:30:00 CST'` by `'1h'` results in
         `'2022-11-06 01:00:00 CST'`, whereas truncating `'2022-11-06 01:30:00 CDT'` by
@@ -192,6 +193,10 @@ def truncate(self, every: str | dt.timedelta | Expr) -> Expr:
         not be 24 hours, due to daylight savings). Similarly for "calendar week",
         "calendar month", "calendar quarter", and "calendar year".
 
+        Durations may not be truncated to a period length `every` containing calendar
+        days, weeks, months, quarters, or years, as these are not constant time
+        intervals.
+
         Returns
         -------
         Expr
@@ -278,15 +283,15 @@ def truncate(self, every: str | dt.timedelta | Expr) -> Expr:
     @unstable()
     def round(self, every: str | dt.timedelta | IntoExprColumn) -> Expr:
         """
-        Divide the date/datetime range into buckets.
+        Divide the dates, datetimes, or durations into buckets.
 
         .. warning::
             This functionality is considered **unstable**. It may be changed
             at any point without it being considered a breaking change.
 
-        Each date/datetime in the first half of the interval
+        Each date/datetime/duration in the first half of the interval
         is mapped to the start of its bucket.
-        Each date/datetime in the second half of the interval
+        Each date/datetime/duration in the second half of the interval
         is mapped to the end of its bucket.
         Ambiguous results are localised using the DST offset of the original timestamp -
         for example, rounding `'2022-11-06 01:20:00 CST'` by `'1h'` results in
@@ -326,6 +331,10 @@ def round(self, every: str | dt.timedelta | IntoExprColumn) -> Expr:
         not be 24 hours, due to daylight savings). Similarly for "calendar week",
         "calendar month", "calendar quarter", and "calendar year".
 
+        Durations may not be rounded to a period length `every` containing calendar
+        days, weeks, months, quarters, or years, as these are not constant time
+        intervals.
+
         Examples
         --------
         >>> from datetime import timedelta, datetime
diff --git a/py-polars/polars/series/datetime.py b/py-polars/polars/series/datetime.py
index 8c8bfb32bad8..de97e54389bd 100644
--- a/py-polars/polars/series/datetime.py
+++ b/py-polars/polars/series/datetime.py
@@ -1644,10 +1644,11 @@ def offset_by(self, by: str | Expr) -> Series:
 
     def truncate(self, every: str | dt.timedelta | IntoExprColumn) -> Series:
         """
-        Divide the date/ datetime range into buckets.
+        Divide the dates, datetimes, or durations into buckets.
 
-        Each date/datetime is mapped to the start of its bucket using the corresponding
-        local datetime. Note that weekly buckets start on Monday.
+        For dates or datetimes, each date/datetime is mapped to the start of its bucket
+        using the corresponding local datetime.
+        Note that weekly buckets start on Monday.
         Ambiguous results are localised using the DST offset of the original timestamp -
         for example, truncating `'2022-11-06 01:30:00 CST'` by `'1h'` results in
         `'2022-11-06 01:00:00 CST'`, whereas truncating `'2022-11-06 01:30:00 CDT'` by
@@ -1683,6 +1684,10 @@ def truncate(self, every: str | dt.timedelta | IntoExprColumn) -> Series:
         not be 24 hours, due to daylight savings). Similarly for "calendar week",
         "calendar month", "calendar quarter", and "calendar year".
 
+        Durations may not be truncated to a period length `every` containing calendar
+        days, weeks, months, quarters, or years, as these are not constant time
+        intervals.
+
         Returns
         -------
         Series
@@ -1758,17 +1763,17 @@ def truncate(self, every: str | dt.timedelta | IntoExprColumn) -> Series:
     @unstable()
     def round(self, every: str | dt.timedelta | IntoExprColumn) -> Series:
         """
-        Divide the date/ datetime range into buckets.
+        Divide the dates, datetimes, or durations into buckets.
 
         .. warning::
             This functionality is considered **unstable**. It may be changed
             at any point without it being considered a breaking change.
 
-        Each date/datetime in the first half of the interval is mapped to the start of
-        its bucket.
-        Each date/datetime in the second half of the interval is mapped to the end of
-        its bucket.
-        Ambiguous results are localized using the DST offset of the original timestamp -
+        Each date/datetime/duration in the first half of the interval
+        is mapped to the start of its bucket.
+        Each date/datetime/duration in the second half of the interval
+        is mapped to the end of its bucket.
+        Ambiguous results are localised using the DST offset of the original timestamp -
         for example, rounding `'2022-11-06 01:20:00 CST'` by `'1h'` results in
         `'2022-11-06 01:00:00 CST'`, whereas rounding `'2022-11-06 01:20:00 CDT'` by
         `'1h'` results in `'2022-11-06 01:00:00 CDT'`.
@@ -1808,6 +1813,10 @@ def round(self, every: str | dt.timedelta | IntoExprColumn) -> Series:
         not be 24 hours, due to daylight savings). Similarly for "calendar week",
         "calendar month", "calendar quarter", and "calendar year".
 
+        Durations may not be rounded to a period length `every` containing calendar
+        days, weeks, months, quarters, or years, as these are not constant time
+        intervals.
+
         Examples
         --------
         >>> from datetime import timedelta, datetime
diff --git a/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py b/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
index c9a43984cd45..0cfdf97ada63 100644
--- a/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
+++ b/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
@@ -486,35 +486,116 @@ def test_truncate(
     assert out.dt[-1] == stop
 
 
+@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
+def test_truncate_duration(time_unit: TimeUnit) -> None:
+    durations = pl.Series(
+        [
+            timedelta(seconds=21),
+            timedelta(seconds=35),
+            timedelta(seconds=59),
+            None,
+            timedelta(seconds=-35),
+        ]
+    ).dt.cast_time_unit(time_unit)
+
+    expected = pl.Series(
+        [
+            timedelta(seconds=20),
+            timedelta(seconds=30),
+            timedelta(seconds=50),
+            None,
+            timedelta(seconds=-30),
+        ]
+    ).dt.cast_time_unit(time_unit)
+
+    assert_series_equal(durations.dt.truncate("10s"), expected)
+
+
+def test_truncate_duration_zero() -> None:
+    """Truncating to the nearest zero should raise a descriptive error."""
+    durations = pl.Series([timedelta(seconds=21), timedelta(seconds=35)])
+
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot truncate a Duration to a non-positive duration",
+    ):
+        durations.dt.truncate("0s")
+
+
+def test_truncate_expressions() -> None:
+    df = pl.DataFrame(
+        {
+            "duration": [
+                timedelta(seconds=20),
+                timedelta(seconds=21),
+                timedelta(seconds=22),
+            ],
+            "every": ["3s", "4s", "5s"],
+        }
+    )
+    result = df.select(pl.col("duration").dt.truncate(pl.col("every")))["duration"]
+    expected = pl.Series(
+        "duration",
+        [timedelta(seconds=18), timedelta(seconds=20), timedelta(seconds=20)],
+    )
+    assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize("every_unit", ["mo", "q", "y"])
+def test_truncated_duration_non_constant(every_unit: str) -> None:
+    # Duration series can't be truncated to non-constant durations
+    df = pl.DataFrame(
+        {
+            "durations": [timedelta(seconds=1), timedelta(seconds=2)],
+            "every": ["1" + every_unit, "1" + every_unit],
+        }
+    )
+
+    with pytest.raises(InvalidOperationError):
+        df["durations"].dt.truncate("1" + every_unit)
+
+    with pytest.raises(InvalidOperationError):
+        df.select(pl.col("durations").dt.truncate(pl.col("every")))
+
+
 def test_truncate_negative() -> None:
     """Test that truncating to a negative duration gives a helpful error message."""
     df = pl.DataFrame(
         {
             "date": [date(1895, 5, 7), date(1955, 11, 5)],
             "datetime": [datetime(1895, 5, 7), datetime(1955, 11, 5)],
-            "duration": ["-1m", "1m"],
+            "duration": [timedelta(minutes=1), timedelta(minutes=-1)],
+            "every": ["-1m", "1m"],
         }
     )
-
     with pytest.raises(
-        ComputeError, match="cannot truncate a Date to a negative duration"
+        InvalidOperationError, match="cannot truncate a Date to a non-positive duration"
     ):
         df.select(pl.col("date").dt.truncate("-1m"))
-
     with pytest.raises(
-        ComputeError, match="cannot truncate a Datetime to a negative duration"
+        InvalidOperationError,
+        match="cannot truncate a Datetime to a non-positive duration",
     ):
         df.select(pl.col("datetime").dt.truncate("-1m"))
-
     with pytest.raises(
-        ComputeError, match="cannot truncate a Date to a negative duration"
+        InvalidOperationError,
+        match="cannot truncate a Duration to a non-positive duration",
+    ):
+        df.select(pl.col("duration").dt.truncate("-1m"))
+    with pytest.raises(
+        InvalidOperationError, match="cannot truncate a Date to a non-positive duration"
     ):
-        df.select(pl.col("date").dt.truncate(pl.col("duration")))
-
+        df.select(pl.col("date").dt.truncate(pl.col("every")))
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot truncate a Datetime to a non-positive duration",
+    ):
+        df.select(pl.col("datetime").dt.truncate(pl.col("every")))
     with pytest.raises(
-        ComputeError, match="cannot truncate a Datetime to a negative duration"
+        InvalidOperationError,
+        match="cannot truncate a Duration to a non-positive duration",
     ):
-        df.select(pl.col("datetime").dt.truncate(pl.col("duration")))
+        df.select(pl.col("duration").dt.truncate(pl.col("every")))
 
 
 @pytest.mark.parametrize(
@@ -551,6 +632,65 @@ def test_round(
     assert out.dt[-1] == stop
 
 
+@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
+def test_round_duration(time_unit: TimeUnit) -> None:
+    durations = pl.Series(
+        [
+            timedelta(seconds=21),
+            timedelta(seconds=35),
+            timedelta(seconds=59),
+            None,
+            timedelta(seconds=-35),
+        ]
+    ).dt.cast_time_unit(time_unit)
+
+    expected = pl.Series(
+        [
+            timedelta(seconds=20),
+            timedelta(seconds=40),
+            timedelta(seconds=60),
+            None,
+            timedelta(seconds=-40),
+        ]
+    ).dt.cast_time_unit(time_unit)
+
+    assert_series_equal(durations.dt.round("10s"), expected)
+
+
+def test_round_duration_zero() -> None:
+    """Rounding to the nearest zero should raise a descriptive error."""
+    durations = pl.Series([timedelta(seconds=21), timedelta(seconds=35)])
+
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot round a Duration to a non-positive duration",
+    ):
+        durations.dt.round("0s")
+
+
+@pytest.mark.parametrize("every", ["mo", "q", "y"])
+def test_round_duration_non_constant(every: str) -> None:
+    # Duration series can't be rounded to non-constant durations
+    durations = pl.Series([timedelta(seconds=21)])
+
+    with pytest.raises(InvalidOperationError):
+        durations.dt.round("1" + every)
+
+
+@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
+def test_round_duration_half(time_unit: TimeUnit) -> None:
+    # Values at halfway points should round away from zero
+    durations = pl.Series(
+        [timedelta(minutes=-30), timedelta(minutes=30), timedelta(minutes=90)]
+    ).dt.cast_time_unit(time_unit)
+
+    expected = pl.Series(
+        [timedelta(hours=-1), timedelta(hours=1), timedelta(hours=2)]
+    ).dt.cast_time_unit(time_unit)
+
+    assert_series_equal(durations.dt.round("1h"), expected)
+
+
 def test_round_expr() -> None:
     df = pl.DataFrame(
         {
@@ -612,15 +752,22 @@ def test_round_expr() -> None:
 def test_round_negative() -> None:
     """Test that rounding to a negative duration gives a helpful error message."""
     with pytest.raises(
-        ComputeError, match="cannot round a Date to a negative duration"
+        InvalidOperationError, match="cannot round a Date to a non-positive duration"
     ):
         pl.Series([date(1895, 5, 7)]).dt.round("-1m")
 
     with pytest.raises(
-        ComputeError, match="cannot round a Datetime to a negative duration"
+        InvalidOperationError,
+        match="cannot round a Datetime to a non-positive duration",
     ):
         pl.Series([datetime(1895, 5, 7)]).dt.round("-1m")
 
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot round a Duration to a non-positive duration",
+    ):
+        pl.Series([timedelta(days=1)]).dt.round("-1m")
+
 
 @pytest.mark.parametrize(
     ("time_unit", "date_in_that_unit"),

From da2199ad6ed4a5f3f3022c22372fa66a53da8708 Mon Sep 17 00:00:00 2001
From: Rob <124158982+rob-sil@users.noreply.github.com>
Date: Tue, 16 Apr 2024 19:02:17 -0700
Subject: [PATCH 2/6] noop


From fde75e9673366bbd6b7fb13ed25006425c7bccee Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 15 Jul 2024 15:56:59 +0100
Subject: [PATCH 3/6] consistency

---
 crates/polars-time/src/round.rs               | 10 ++++-----
 crates/polars-time/src/truncate.rs            | 12 +++++-----
 .../namespaces/temporal/test_datetime.py      | 22 +++++++++----------
 3 files changed, 22 insertions(+), 22 deletions(-)

diff --git a/crates/polars-time/src/round.rs b/crates/polars-time/src/round.rs
index ab0878c3d40d..f8b6d2a6201c 100644
--- a/crates/polars-time/src/round.rs
+++ b/crates/polars-time/src/round.rs
@@ -21,7 +21,7 @@ impl PolarsRound for DatetimeChunked {
         if every.len() == 1 {
             if let Some(every) = every.get(0) {
                 let every_parsed = Duration::parse(every);
-                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Datetime to a non-positive duration");
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Datetime to a non-positive Duration");
                 if (time_zone.is_none() || time_zone.as_deref() == Some("UTC"))
                     && (every_parsed.months() == 0 && every_parsed.weeks() == 0)
                 {
@@ -76,7 +76,7 @@ impl PolarsRound for DatetimeChunked {
             (Some(timestamp), Some(every)) => {
                 let every_parsed =
                     *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
-                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Date to a non-positive duration");
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Datetime to a non-positive Duration");
 
                 let w = Window::new(every_parsed, every_parsed, offset);
                 func(&w, timestamp, tz).map(Some)
@@ -94,7 +94,7 @@ impl PolarsRound for DateChunked {
             1 => {
                 if let Some(every) = every.get(0) {
                     let every_parsed = Duration::parse(every);
-                    polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Date to a non-positive duration");
+                    polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Date to a non-positive Duration");
                     let w = Window::new(every_parsed, every_parsed, offset);
                     self.try_apply_nonnull_values_generic(|t| {
                         Ok(
@@ -113,7 +113,7 @@ impl PolarsRound for DateChunked {
                     (Some(t), Some(every)) => {
                         let every_parsed = *duration_cache
                             .get_or_insert_with(every, |every| Duration::parse(every));
-                        polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Date to a non-positive duration");
+                        polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Date to a non-positive Duration");
 
                         let w = Window::new(every_parsed, every_parsed, offset);
                         Ok(Some(
@@ -135,7 +135,7 @@ impl PolarsRound for DurationChunked {
         if every.len() == 1 {
             if let Some(every) = every.get(0) {
                 let every_parsed = Duration::parse(every);
-                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Duration to a non-positive duration");
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Duration to a non-positive Duration");
                 polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot round a Duration to a non-constant duration (i.e. one that involves weeks / months)");
                 let every = match self.time_unit() {
                     TimeUnit::Milliseconds => every_parsed.duration_ms(),
diff --git a/crates/polars-time/src/truncate.rs b/crates/polars-time/src/truncate.rs
index a4cf11ab47e1..cbb260756157 100644
--- a/crates/polars-time/src/truncate.rs
+++ b/crates/polars-time/src/truncate.rs
@@ -21,7 +21,7 @@ impl PolarsTruncate for DatetimeChunked {
         if every.len() == 1 {
             if let Some(every) = every.get(0) {
                 let every_parsed = Duration::parse(every);
-                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Datetime to a non-positive duration");
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Datetime to a non-positive Duration");
                 if (time_zone.is_none() || time_zone.as_deref() == Some("UTC"))
                     && (every_parsed.months() == 0 && every_parsed.weeks() == 0)
                 {
@@ -75,7 +75,7 @@ impl PolarsTruncate for DatetimeChunked {
             (Some(timestamp), Some(every)) => {
                 let every_parsed =
                     *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
-                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Datetime to a non-positive duration");
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Datetime to a non-positive Duration");
 
                 let w = Window::new(every_parsed, every_parsed, offset);
                 func(&w, timestamp, tz).map(Some)
@@ -93,7 +93,7 @@ impl PolarsTruncate for DateChunked {
             1 => {
                 if let Some(every) = every.get(0) {
                     let every_parsed = Duration::parse(every);
-                    polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Date to a non-positive duration");
+                    polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Date to a non-positive Duration");
                     let w = Window::new(every_parsed, every_parsed, offset);
                     self.try_apply_nonnull_values_generic(|t| {
                         Ok((w.truncate_ms(MILLISECONDS_IN_DAY * t as i64, None)?
@@ -110,7 +110,7 @@ impl PolarsTruncate for DateChunked {
                     (Some(t), Some(every)) => {
                         let every_parsed = *duration_cache
                             .get_or_insert_with(every, |every| Duration::parse(every));
-                        polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Date to a non-positive duration");
+                        polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Date to a non-positive Duration");
 
                         let w = Window::new(every_parsed, every_parsed, offset);
                         Ok(Some(
@@ -132,7 +132,7 @@ impl PolarsTruncate for DurationChunked {
         if every.len() == 1 {
             if let Some(every) = every.get(0) {
                 let every_parsed = Duration::parse(every);
-                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Duration to a non-positive duration");
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Duration to a non-positive Duration");
                 polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot truncate a Duration to a non-constant duration (i.e. one that involves weeks / months)");
                 let every = match self.time_unit() {
                     TimeUnit::Milliseconds => every_parsed.duration_ms(),
@@ -158,7 +158,7 @@ impl PolarsTruncate for DurationChunked {
             (Some(t), Some(every)) => {
                 let every_parsed =
                     *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
-                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Duration to a non-positive duration");
+                polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Duration to a non-positive Duration");
                 polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot truncate a Duration to a non-constant duration (i.e. one that involves weeks / months)");
                 let every = match self.time_unit() {
                     TimeUnit::Milliseconds => every_parsed.duration_ms(),
diff --git a/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py b/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
index 0cfdf97ada63..fdfaef04893b 100644
--- a/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
+++ b/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
@@ -517,7 +517,7 @@ def test_truncate_duration_zero() -> None:
 
     with pytest.raises(
         InvalidOperationError,
-        match="cannot truncate a Duration to a non-positive duration",
+        match="cannot truncate a Duration to a non-positive Duration",
     ):
         durations.dt.truncate("0s")
 
@@ -569,31 +569,31 @@ def test_truncate_negative() -> None:
         }
     )
     with pytest.raises(
-        InvalidOperationError, match="cannot truncate a Date to a non-positive duration"
+        InvalidOperationError, match="cannot truncate a Date to a non-positive Duration"
     ):
         df.select(pl.col("date").dt.truncate("-1m"))
     with pytest.raises(
         InvalidOperationError,
-        match="cannot truncate a Datetime to a non-positive duration",
+        match="cannot truncate a Datetime to a non-positive Duration",
     ):
         df.select(pl.col("datetime").dt.truncate("-1m"))
     with pytest.raises(
         InvalidOperationError,
-        match="cannot truncate a Duration to a non-positive duration",
+        match="cannot truncate a Duration to a non-positive Duration",
     ):
         df.select(pl.col("duration").dt.truncate("-1m"))
     with pytest.raises(
-        InvalidOperationError, match="cannot truncate a Date to a non-positive duration"
+        InvalidOperationError, match="cannot truncate a Date to a non-positive Duration"
     ):
         df.select(pl.col("date").dt.truncate(pl.col("every")))
     with pytest.raises(
         InvalidOperationError,
-        match="cannot truncate a Datetime to a non-positive duration",
+        match="cannot truncate a Datetime to a non-positive Duration",
     ):
         df.select(pl.col("datetime").dt.truncate(pl.col("every")))
     with pytest.raises(
         InvalidOperationError,
-        match="cannot truncate a Duration to a non-positive duration",
+        match="cannot truncate a Duration to a non-positive Duration",
     ):
         df.select(pl.col("duration").dt.truncate(pl.col("every")))
 
@@ -663,7 +663,7 @@ def test_round_duration_zero() -> None:
 
     with pytest.raises(
         InvalidOperationError,
-        match="cannot round a Duration to a non-positive duration",
+        match="cannot round a Duration to a non-positive Duration",
     ):
         durations.dt.round("0s")
 
@@ -752,19 +752,19 @@ def test_round_expr() -> None:
 def test_round_negative() -> None:
     """Test that rounding to a negative duration gives a helpful error message."""
     with pytest.raises(
-        InvalidOperationError, match="cannot round a Date to a non-positive duration"
+        InvalidOperationError, match="cannot round a Date to a non-positive Duration"
     ):
         pl.Series([date(1895, 5, 7)]).dt.round("-1m")
 
     with pytest.raises(
         InvalidOperationError,
-        match="cannot round a Datetime to a non-positive duration",
+        match="cannot round a Datetime to a non-positive Duration",
     ):
         pl.Series([datetime(1895, 5, 7)]).dt.round("-1m")
 
     with pytest.raises(
         InvalidOperationError,
-        match="cannot round a Duration to a non-positive duration",
+        match="cannot round a Duration to a non-positive Duration",
     ):
         pl.Series([timedelta(days=1)]).dt.round("-1m")
 

From fc7a3c782a8afa45eff293868b375c3c38327271 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 15 Jul 2024 16:00:38 +0100
Subject: [PATCH 4/6] consistency

---
 crates/polars-time/src/round.rs    | 4 ++--
 crates/polars-time/src/truncate.rs | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/crates/polars-time/src/round.rs b/crates/polars-time/src/round.rs
index f8b6d2a6201c..8a304958db32 100644
--- a/crates/polars-time/src/round.rs
+++ b/crates/polars-time/src/round.rs
@@ -136,7 +136,7 @@ impl PolarsRound for DurationChunked {
             if let Some(every) = every.get(0) {
                 let every_parsed = Duration::parse(every);
                 polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot round a Duration to a non-positive Duration");
-                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot round a Duration to a non-constant duration (i.e. one that involves weeks / months)");
+                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot round a Duration to a non-constant Duration (i.e. one that involves weeks / months)");
                 let every = match self.time_unit() {
                     TimeUnit::Milliseconds => every_parsed.duration_ms(),
                     TimeUnit::Microseconds => every_parsed.duration_us(),
@@ -166,7 +166,7 @@ impl PolarsRound for DurationChunked {
                 let every_parsed =
                     *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
                 polars_ensure!(!every_parsed.negative, InvalidOperation: "cannot round a Duration to a negative duration");
-                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot round a Duration to a non-constant duration (i.e. one that involves weeks / months)");
+                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot round a Duration to a non-constant Duration (i.e. one that involves weeks / months)");
                 let every = match self.time_unit() {
                     TimeUnit::Milliseconds => every_parsed.duration_ms(),
                     TimeUnit::Microseconds => every_parsed.duration_us(),
diff --git a/crates/polars-time/src/truncate.rs b/crates/polars-time/src/truncate.rs
index cbb260756157..b7380378e1cd 100644
--- a/crates/polars-time/src/truncate.rs
+++ b/crates/polars-time/src/truncate.rs
@@ -133,7 +133,7 @@ impl PolarsTruncate for DurationChunked {
             if let Some(every) = every.get(0) {
                 let every_parsed = Duration::parse(every);
                 polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Duration to a non-positive Duration");
-                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot truncate a Duration to a non-constant duration (i.e. one that involves weeks / months)");
+                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot truncate a Duration to a non-constant Duration (i.e. one that involves weeks / months)");
                 let every = match self.time_unit() {
                     TimeUnit::Milliseconds => every_parsed.duration_ms(),
                     TimeUnit::Microseconds => every_parsed.duration_us(),
@@ -159,7 +159,7 @@ impl PolarsTruncate for DurationChunked {
                 let every_parsed =
                     *duration_cache.get_or_insert_with(every, |every| Duration::parse(every));
                 polars_ensure!(!every_parsed.negative & !every_parsed.is_zero(), InvalidOperation: "cannot truncate a Duration to a non-positive Duration");
-                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot truncate a Duration to a non-constant duration (i.e. one that involves weeks / months)");
+                polars_ensure!(every_parsed.is_constant_duration(None), InvalidOperation:"cannot truncate a Duration to a non-constant Duration (i.e. one that involves weeks / months)");
                 let every = match self.time_unit() {
                     TimeUnit::Milliseconds => every_parsed.duration_ms(),
                     TimeUnit::Microseconds => every_parsed.duration_us(),

From c72d3c48d370cfd3b02247a93e4590c4a5b461de Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 15 Jul 2024 16:02:52 +0100
Subject: [PATCH 5/6] move tests to more appropriate location

---
 .../namespaces/temporal/test_datetime.py      | 317 ------------------
 .../namespaces/temporal/test_round.py         | 174 +++++++++-
 .../namespaces/temporal/test_truncate.py      | 147 ++++++++
 3 files changed, 320 insertions(+), 318 deletions(-)

diff --git a/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py b/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
index fdfaef04893b..0329400012b9 100644
--- a/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
+++ b/py-polars/tests/unit/operations/namespaces/temporal/test_datetime.py
@@ -452,323 +452,6 @@ def test_duration_extract_times(
     assert_series_equal(getattr(duration.dt, unit_attr)(), expected)
 
 
-@pytest.mark.parametrize(
-    ("time_unit", "every"),
-    [
-        ("ms", "1h"),
-        ("us", "1h0m0s"),
-        ("ns", timedelta(hours=1)),
-    ],
-    ids=["milliseconds", "microseconds", "nanoseconds"],
-)
-def test_truncate(
-    time_unit: TimeUnit,
-    every: str | timedelta,
-) -> None:
-    start, stop = datetime(2022, 1, 1), datetime(2022, 1, 2)
-    s = pl.datetime_range(
-        start,
-        stop,
-        timedelta(minutes=30),
-        time_unit=time_unit,
-        eager=True,
-    ).alias(f"dates[{time_unit}]")
-
-    # can pass strings and time-deltas
-    out = s.dt.truncate(every)
-    assert out.dt[0] == start
-    assert out.dt[1] == start
-    assert out.dt[2] == start + timedelta(hours=1)
-    assert out.dt[3] == start + timedelta(hours=1)
-    # ...
-    assert out.dt[-3] == stop - timedelta(hours=1)
-    assert out.dt[-2] == stop - timedelta(hours=1)
-    assert out.dt[-1] == stop
-
-
-@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
-def test_truncate_duration(time_unit: TimeUnit) -> None:
-    durations = pl.Series(
-        [
-            timedelta(seconds=21),
-            timedelta(seconds=35),
-            timedelta(seconds=59),
-            None,
-            timedelta(seconds=-35),
-        ]
-    ).dt.cast_time_unit(time_unit)
-
-    expected = pl.Series(
-        [
-            timedelta(seconds=20),
-            timedelta(seconds=30),
-            timedelta(seconds=50),
-            None,
-            timedelta(seconds=-30),
-        ]
-    ).dt.cast_time_unit(time_unit)
-
-    assert_series_equal(durations.dt.truncate("10s"), expected)
-
-
-def test_truncate_duration_zero() -> None:
-    """Truncating to the nearest zero should raise a descriptive error."""
-    durations = pl.Series([timedelta(seconds=21), timedelta(seconds=35)])
-
-    with pytest.raises(
-        InvalidOperationError,
-        match="cannot truncate a Duration to a non-positive Duration",
-    ):
-        durations.dt.truncate("0s")
-
-
-def test_truncate_expressions() -> None:
-    df = pl.DataFrame(
-        {
-            "duration": [
-                timedelta(seconds=20),
-                timedelta(seconds=21),
-                timedelta(seconds=22),
-            ],
-            "every": ["3s", "4s", "5s"],
-        }
-    )
-    result = df.select(pl.col("duration").dt.truncate(pl.col("every")))["duration"]
-    expected = pl.Series(
-        "duration",
-        [timedelta(seconds=18), timedelta(seconds=20), timedelta(seconds=20)],
-    )
-    assert_series_equal(result, expected)
-
-
-@pytest.mark.parametrize("every_unit", ["mo", "q", "y"])
-def test_truncated_duration_non_constant(every_unit: str) -> None:
-    # Duration series can't be truncated to non-constant durations
-    df = pl.DataFrame(
-        {
-            "durations": [timedelta(seconds=1), timedelta(seconds=2)],
-            "every": ["1" + every_unit, "1" + every_unit],
-        }
-    )
-
-    with pytest.raises(InvalidOperationError):
-        df["durations"].dt.truncate("1" + every_unit)
-
-    with pytest.raises(InvalidOperationError):
-        df.select(pl.col("durations").dt.truncate(pl.col("every")))
-
-
-def test_truncate_negative() -> None:
-    """Test that truncating to a negative duration gives a helpful error message."""
-    df = pl.DataFrame(
-        {
-            "date": [date(1895, 5, 7), date(1955, 11, 5)],
-            "datetime": [datetime(1895, 5, 7), datetime(1955, 11, 5)],
-            "duration": [timedelta(minutes=1), timedelta(minutes=-1)],
-            "every": ["-1m", "1m"],
-        }
-    )
-    with pytest.raises(
-        InvalidOperationError, match="cannot truncate a Date to a non-positive Duration"
-    ):
-        df.select(pl.col("date").dt.truncate("-1m"))
-    with pytest.raises(
-        InvalidOperationError,
-        match="cannot truncate a Datetime to a non-positive Duration",
-    ):
-        df.select(pl.col("datetime").dt.truncate("-1m"))
-    with pytest.raises(
-        InvalidOperationError,
-        match="cannot truncate a Duration to a non-positive Duration",
-    ):
-        df.select(pl.col("duration").dt.truncate("-1m"))
-    with pytest.raises(
-        InvalidOperationError, match="cannot truncate a Date to a non-positive Duration"
-    ):
-        df.select(pl.col("date").dt.truncate(pl.col("every")))
-    with pytest.raises(
-        InvalidOperationError,
-        match="cannot truncate a Datetime to a non-positive Duration",
-    ):
-        df.select(pl.col("datetime").dt.truncate(pl.col("every")))
-    with pytest.raises(
-        InvalidOperationError,
-        match="cannot truncate a Duration to a non-positive Duration",
-    ):
-        df.select(pl.col("duration").dt.truncate(pl.col("every")))
-
-
-@pytest.mark.parametrize(
-    ("time_unit", "every"),
-    [
-        ("ms", "1h"),
-        ("us", "1h0m0s"),
-        ("ns", timedelta(hours=1)),
-    ],
-    ids=["milliseconds", "microseconds", "nanoseconds"],
-)
-def test_round(
-    time_unit: TimeUnit,
-    every: str | timedelta,
-) -> None:
-    start, stop = datetime(2022, 1, 1), datetime(2022, 1, 2)
-    s = pl.datetime_range(
-        start,
-        stop,
-        timedelta(minutes=30),
-        time_unit=time_unit,
-        eager=True,
-    ).alias(f"dates[{time_unit}]")
-
-    # can pass strings and time-deltas
-    out = s.dt.round(every)
-    assert out.dt[0] == start
-    assert out.dt[1] == start + timedelta(hours=1)
-    assert out.dt[2] == start + timedelta(hours=1)
-    assert out.dt[3] == start + timedelta(hours=2)
-    # ...
-    assert out.dt[-3] == stop - timedelta(hours=1)
-    assert out.dt[-2] == stop
-    assert out.dt[-1] == stop
-
-
-@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
-def test_round_duration(time_unit: TimeUnit) -> None:
-    durations = pl.Series(
-        [
-            timedelta(seconds=21),
-            timedelta(seconds=35),
-            timedelta(seconds=59),
-            None,
-            timedelta(seconds=-35),
-        ]
-    ).dt.cast_time_unit(time_unit)
-
-    expected = pl.Series(
-        [
-            timedelta(seconds=20),
-            timedelta(seconds=40),
-            timedelta(seconds=60),
-            None,
-            timedelta(seconds=-40),
-        ]
-    ).dt.cast_time_unit(time_unit)
-
-    assert_series_equal(durations.dt.round("10s"), expected)
-
-
-def test_round_duration_zero() -> None:
-    """Rounding to the nearest zero should raise a descriptive error."""
-    durations = pl.Series([timedelta(seconds=21), timedelta(seconds=35)])
-
-    with pytest.raises(
-        InvalidOperationError,
-        match="cannot round a Duration to a non-positive Duration",
-    ):
-        durations.dt.round("0s")
-
-
-@pytest.mark.parametrize("every", ["mo", "q", "y"])
-def test_round_duration_non_constant(every: str) -> None:
-    # Duration series can't be rounded to non-constant durations
-    durations = pl.Series([timedelta(seconds=21)])
-
-    with pytest.raises(InvalidOperationError):
-        durations.dt.round("1" + every)
-
-
-@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
-def test_round_duration_half(time_unit: TimeUnit) -> None:
-    # Values at halfway points should round away from zero
-    durations = pl.Series(
-        [timedelta(minutes=-30), timedelta(minutes=30), timedelta(minutes=90)]
-    ).dt.cast_time_unit(time_unit)
-
-    expected = pl.Series(
-        [timedelta(hours=-1), timedelta(hours=1), timedelta(hours=2)]
-    ).dt.cast_time_unit(time_unit)
-
-    assert_series_equal(durations.dt.round("1h"), expected)
-
-
-def test_round_expr() -> None:
-    df = pl.DataFrame(
-        {
-            "date": [
-                datetime(2022, 11, 14),
-                datetime(2023, 10, 11),
-                datetime(2022, 3, 20, 5, 7, 18),
-                datetime(2022, 4, 3, 13, 30, 32),
-                None,
-                datetime(2022, 12, 1),
-            ],
-            "every": ["1y", "1mo", "1m", "1m", "1mo", None],
-        }
-    )
-
-    output = df.select(
-        all_expr=pl.col("date").dt.round(every=pl.col("every")),
-        date_lit=pl.lit(datetime(2022, 4, 3, 13, 30, 32)).dt.round(
-            every=pl.col("every")
-        ),
-        every_lit=pl.col("date").dt.round("1d"),
-    )
-
-    expected = pl.DataFrame(
-        {
-            "all_expr": [
-                datetime(2023, 1, 1),
-                datetime(2023, 10, 1),
-                datetime(2022, 3, 20, 5, 7),
-                datetime(2022, 4, 3, 13, 31),
-                None,
-                None,
-            ],
-            "date_lit": [
-                datetime(2022, 1, 1),
-                datetime(2022, 4, 1),
-                datetime(2022, 4, 3, 13, 31),
-                datetime(2022, 4, 3, 13, 31),
-                datetime(2022, 4, 1),
-                None,
-            ],
-            "every_lit": [
-                datetime(2022, 11, 14),
-                datetime(2023, 10, 11),
-                datetime(2022, 3, 20),
-                datetime(2022, 4, 4),
-                None,
-                datetime(2022, 12, 1),
-            ],
-        }
-    )
-
-    assert_frame_equal(output, expected)
-
-    all_lit = pl.select(all_lit=pl.lit(datetime(2022, 3, 20, 5, 7)).dt.round("1h"))
-    assert all_lit.to_dict(as_series=False) == {"all_lit": [datetime(2022, 3, 20, 5)]}
-
-
-def test_round_negative() -> None:
-    """Test that rounding to a negative duration gives a helpful error message."""
-    with pytest.raises(
-        InvalidOperationError, match="cannot round a Date to a non-positive Duration"
-    ):
-        pl.Series([date(1895, 5, 7)]).dt.round("-1m")
-
-    with pytest.raises(
-        InvalidOperationError,
-        match="cannot round a Datetime to a non-positive Duration",
-    ):
-        pl.Series([datetime(1895, 5, 7)]).dt.round("-1m")
-
-    with pytest.raises(
-        InvalidOperationError,
-        match="cannot round a Duration to a non-positive Duration",
-    ):
-        pl.Series([timedelta(days=1)]).dt.round("-1m")
-
-
 @pytest.mark.parametrize(
     ("time_unit", "date_in_that_unit"),
     [
diff --git a/py-polars/tests/unit/operations/namespaces/temporal/test_round.py b/py-polars/tests/unit/operations/namespaces/temporal/test_round.py
index 1ac7acc3edcd..25b7304208cf 100644
--- a/py-polars/tests/unit/operations/namespaces/temporal/test_round.py
+++ b/py-polars/tests/unit/operations/namespaces/temporal/test_round.py
@@ -9,7 +9,8 @@
 
 import polars as pl
 from polars._utils.convert import parse_as_duration_string
-from polars.testing import assert_series_equal
+from polars.exceptions import InvalidOperationError
+from polars.testing import assert_frame_equal, assert_series_equal
 
 if TYPE_CHECKING:
     from zoneinfo import ZoneInfo
@@ -189,3 +190,174 @@ def test_round_datetime_w_expression(time_unit: TimeUnit) -> None:
     result = df.select(pl.col("a").dt.round(pl.col("b")))["a"]
     assert result[0] == datetime(2020, 1, 1)
     assert result[1] == datetime(2020, 1, 21)
+
+
+@pytest.mark.parametrize(
+    ("time_unit", "every"),
+    [
+        ("ms", "1h"),
+        ("us", "1h0m0s"),
+        ("ns", timedelta(hours=1)),
+    ],
+    ids=["milliseconds", "microseconds", "nanoseconds"],
+)
+def test_round(
+    time_unit: TimeUnit,
+    every: str | timedelta,
+) -> None:
+    start, stop = datetime(2022, 1, 1), datetime(2022, 1, 2)
+    s = pl.datetime_range(
+        start,
+        stop,
+        timedelta(minutes=30),
+        time_unit=time_unit,
+        eager=True,
+    ).alias(f"dates[{time_unit}]")
+
+    # can pass strings and time-deltas
+    out = s.dt.round(every)
+    assert out.dt[0] == start
+    assert out.dt[1] == start + timedelta(hours=1)
+    assert out.dt[2] == start + timedelta(hours=1)
+    assert out.dt[3] == start + timedelta(hours=2)
+    # ...
+    assert out.dt[-3] == stop - timedelta(hours=1)
+    assert out.dt[-2] == stop
+    assert out.dt[-1] == stop
+
+
+@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
+def test_round_duration(time_unit: TimeUnit) -> None:
+    durations = pl.Series(
+        [
+            timedelta(seconds=21),
+            timedelta(seconds=35),
+            timedelta(seconds=59),
+            None,
+            timedelta(seconds=-35),
+        ]
+    ).dt.cast_time_unit(time_unit)
+
+    expected = pl.Series(
+        [
+            timedelta(seconds=20),
+            timedelta(seconds=40),
+            timedelta(seconds=60),
+            None,
+            timedelta(seconds=-40),
+        ]
+    ).dt.cast_time_unit(time_unit)
+
+    assert_series_equal(durations.dt.round("10s"), expected)
+
+
+def test_round_duration_zero() -> None:
+    """Rounding to the nearest zero should raise a descriptive error."""
+    durations = pl.Series([timedelta(seconds=21), timedelta(seconds=35)])
+
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot round a Duration to a non-positive Duration",
+    ):
+        durations.dt.round("0s")
+
+
+@pytest.mark.parametrize("every", ["mo", "q", "y"])
+def test_round_duration_non_constant(every: str) -> None:
+    # Duration series can't be rounded to non-constant durations
+    durations = pl.Series([timedelta(seconds=21)])
+
+    with pytest.raises(InvalidOperationError):
+        durations.dt.round("1" + every)
+
+
+@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
+def test_round_duration_half(time_unit: TimeUnit) -> None:
+    # Values at halfway points should round away from zero
+    durations = pl.Series(
+        [timedelta(minutes=-30), timedelta(minutes=30), timedelta(minutes=90)]
+    ).dt.cast_time_unit(time_unit)
+
+    expected = pl.Series(
+        [timedelta(hours=-1), timedelta(hours=1), timedelta(hours=2)]
+    ).dt.cast_time_unit(time_unit)
+
+    assert_series_equal(durations.dt.round("1h"), expected)
+
+
+def test_round_expr() -> None:
+    df = pl.DataFrame(
+        {
+            "date": [
+                datetime(2022, 11, 14),
+                datetime(2023, 10, 11),
+                datetime(2022, 3, 20, 5, 7, 18),
+                datetime(2022, 4, 3, 13, 30, 32),
+                None,
+                datetime(2022, 12, 1),
+            ],
+            "every": ["1y", "1mo", "1m", "1m", "1mo", None],
+        }
+    )
+
+    output = df.select(
+        all_expr=pl.col("date").dt.round(every=pl.col("every")),
+        date_lit=pl.lit(datetime(2022, 4, 3, 13, 30, 32)).dt.round(
+            every=pl.col("every")
+        ),
+        every_lit=pl.col("date").dt.round("1d"),
+    )
+
+    expected = pl.DataFrame(
+        {
+            "all_expr": [
+                datetime(2023, 1, 1),
+                datetime(2023, 10, 1),
+                datetime(2022, 3, 20, 5, 7),
+                datetime(2022, 4, 3, 13, 31),
+                None,
+                None,
+            ],
+            "date_lit": [
+                datetime(2022, 1, 1),
+                datetime(2022, 4, 1),
+                datetime(2022, 4, 3, 13, 31),
+                datetime(2022, 4, 3, 13, 31),
+                datetime(2022, 4, 1),
+                None,
+            ],
+            "every_lit": [
+                datetime(2022, 11, 14),
+                datetime(2023, 10, 11),
+                datetime(2022, 3, 20),
+                datetime(2022, 4, 4),
+                None,
+                datetime(2022, 12, 1),
+            ],
+        }
+    )
+
+    assert_frame_equal(output, expected)
+
+    all_lit = pl.select(all_lit=pl.lit(datetime(2022, 3, 20, 5, 7)).dt.round("1h"))
+    assert all_lit.to_dict(as_series=False) == {"all_lit": [datetime(2022, 3, 20, 5)]}
+
+
+def test_round_negative() -> None:
+    """Test that rounding to a negative duration gives a helpful error message."""
+    with pytest.raises(
+        InvalidOperationError, match="cannot round a Date to a non-positive Duration"
+    ):
+        pl.Series([date(1895, 5, 7)]).dt.round("-1m")
+
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot round a Datetime to a non-positive Duration",
+    ):
+        pl.Series([datetime(1895, 5, 7)]).dt.round("-1m")
+
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot round a Duration to a non-positive Duration",
+    ):
+        pl.Series([timedelta(days=1)]).dt.round("-1m")
diff --git a/py-polars/tests/unit/operations/namespaces/temporal/test_truncate.py b/py-polars/tests/unit/operations/namespaces/temporal/test_truncate.py
index f56d356b0457..7a7fee428d5b 100644
--- a/py-polars/tests/unit/operations/namespaces/temporal/test_truncate.py
+++ b/py-polars/tests/unit/operations/namespaces/temporal/test_truncate.py
@@ -9,6 +9,7 @@
 
 import polars as pl
 from polars._utils.convert import parse_as_duration_string
+from polars.exceptions import InvalidOperationError
 from polars.testing import assert_series_equal
 
 if TYPE_CHECKING:
@@ -119,3 +120,149 @@ def test_fast_path_vs_slow_path(datetimes: list[datetime], every: str) -> None:
     # Definitely uses slowpath:
     expected = s.dt.truncate(pl.Series([every] * len(datetimes)))
     assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    ("time_unit", "every"),
+    [
+        ("ms", "1h"),
+        ("us", "1h0m0s"),
+        ("ns", timedelta(hours=1)),
+    ],
+    ids=["milliseconds", "microseconds", "nanoseconds"],
+)
+def test_truncate(
+    time_unit: TimeUnit,
+    every: str | timedelta,
+) -> None:
+    start, stop = datetime(2022, 1, 1), datetime(2022, 1, 2)
+    s = pl.datetime_range(
+        start,
+        stop,
+        timedelta(minutes=30),
+        time_unit=time_unit,
+        eager=True,
+    ).alias(f"dates[{time_unit}]")
+
+    # can pass strings and time-deltas
+    out = s.dt.truncate(every)
+    assert out.dt[0] == start
+    assert out.dt[1] == start
+    assert out.dt[2] == start + timedelta(hours=1)
+    assert out.dt[3] == start + timedelta(hours=1)
+    # ...
+    assert out.dt[-3] == stop - timedelta(hours=1)
+    assert out.dt[-2] == stop - timedelta(hours=1)
+    assert out.dt[-1] == stop
+
+
+@pytest.mark.parametrize("time_unit", ["ms", "us", "ns"])
+def test_truncate_duration(time_unit: TimeUnit) -> None:
+    durations = pl.Series(
+        [
+            timedelta(seconds=21),
+            timedelta(seconds=35),
+            timedelta(seconds=59),
+            None,
+            timedelta(seconds=-35),
+        ]
+    ).dt.cast_time_unit(time_unit)
+
+    expected = pl.Series(
+        [
+            timedelta(seconds=20),
+            timedelta(seconds=30),
+            timedelta(seconds=50),
+            None,
+            timedelta(seconds=-30),
+        ]
+    ).dt.cast_time_unit(time_unit)
+
+    assert_series_equal(durations.dt.truncate("10s"), expected)
+
+
+def test_truncate_duration_zero() -> None:
+    """Truncating to the nearest zero should raise a descriptive error."""
+    durations = pl.Series([timedelta(seconds=21), timedelta(seconds=35)])
+
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot truncate a Duration to a non-positive Duration",
+    ):
+        durations.dt.truncate("0s")
+
+
+def test_truncate_expressions() -> None:
+    df = pl.DataFrame(
+        {
+            "duration": [
+                timedelta(seconds=20),
+                timedelta(seconds=21),
+                timedelta(seconds=22),
+            ],
+            "every": ["3s", "4s", "5s"],
+        }
+    )
+    result = df.select(pl.col("duration").dt.truncate(pl.col("every")))["duration"]
+    expected = pl.Series(
+        "duration",
+        [timedelta(seconds=18), timedelta(seconds=20), timedelta(seconds=20)],
+    )
+    assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize("every_unit", ["mo", "q", "y"])
+def test_truncated_duration_non_constant(every_unit: str) -> None:
+    # Duration series can't be truncated to non-constant durations
+    df = pl.DataFrame(
+        {
+            "durations": [timedelta(seconds=1), timedelta(seconds=2)],
+            "every": ["1" + every_unit, "1" + every_unit],
+        }
+    )
+
+    with pytest.raises(InvalidOperationError):
+        df["durations"].dt.truncate("1" + every_unit)
+
+    with pytest.raises(InvalidOperationError):
+        df.select(pl.col("durations").dt.truncate(pl.col("every")))
+
+
+def test_truncate_negative() -> None:
+    """Test that truncating to a negative duration gives a helpful error message."""
+    df = pl.DataFrame(
+        {
+            "date": [date(1895, 5, 7), date(1955, 11, 5)],
+            "datetime": [datetime(1895, 5, 7), datetime(1955, 11, 5)],
+            "duration": [timedelta(minutes=1), timedelta(minutes=-1)],
+            "every": ["-1m", "1m"],
+        }
+    )
+    with pytest.raises(
+        InvalidOperationError, match="cannot truncate a Date to a non-positive Duration"
+    ):
+        df.select(pl.col("date").dt.truncate("-1m"))
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot truncate a Datetime to a non-positive Duration",
+    ):
+        df.select(pl.col("datetime").dt.truncate("-1m"))
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot truncate a Duration to a non-positive Duration",
+    ):
+        df.select(pl.col("duration").dt.truncate("-1m"))
+    with pytest.raises(
+        InvalidOperationError, match="cannot truncate a Date to a non-positive Duration"
+    ):
+        df.select(pl.col("date").dt.truncate(pl.col("every")))
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot truncate a Datetime to a non-positive Duration",
+    ):
+        df.select(pl.col("datetime").dt.truncate(pl.col("every")))
+    with pytest.raises(
+        InvalidOperationError,
+        match="cannot truncate a Duration to a non-positive Duration",
+    ):
+        df.select(pl.col("duration").dt.truncate(pl.col("every")))

From 1554a579a9464d9f2e3d4e84435bd1e5fe927c22 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 15 Jul 2024 17:31:47 +0100
Subject: [PATCH 6/6] align formulae between datetime and duration

---
 crates/polars-time/src/round.rs               | 21 +++++++------------
 crates/polars-time/src/truncate.rs            | 14 +++++++------
 .../namespaces/temporal/test_truncate.py      |  2 +-
 3 files changed, 17 insertions(+), 20 deletions(-)

diff --git a/crates/polars-time/src/round.rs b/crates/polars-time/src/round.rs
index 8a304958db32..5821f3e786fb 100644
--- a/crates/polars-time/src/round.rs
+++ b/crates/polars-time/src/round.rs
@@ -12,6 +12,11 @@ pub trait PolarsRound {
         Self: Sized;
 }
 
+fn simple_round(t: i64, every: i64) -> i64 {
+    let half_away = t.signum() * every / 2;
+    t + half_away - (t + half_away) % every
+}
+
 impl PolarsRound for DatetimeChunked {
     fn round(&self, every: &StringChunked, tz: Option<&Tz>) -> PolarsResult<Self> {
         let time_zone = self.time_zone();
@@ -33,11 +38,7 @@ impl PolarsRound for DatetimeChunked {
                         TimeUnit::Nanoseconds => every_parsed.duration_ns(),
                     };
                     return Ok(self
-                        .apply_values(|t| {
-                            // Round half-way values away from zero
-                            let half_away = t.signum() * every / 2;
-                            t + half_away - (t + half_away) % every
-                        })
+                        .apply_values(|t| simple_round(t, every))
                         .into_datetime(self.time_unit(), time_zone.clone()));
                 } else {
                     let w = Window::new(every_parsed, every_parsed, offset);
@@ -143,11 +144,7 @@ impl PolarsRound for DurationChunked {
                     TimeUnit::Nanoseconds => every_parsed.duration_ns(),
                 };
                 return Ok(self
-                    .apply_values(|t| {
-                        // Round half-way values away from zero
-                        let half_away = t.signum() * every / 2;
-                        t + half_away - (t + half_away) % every
-                    })
+                    .apply_values(|t| simple_round(t, every))
                     .into_duration(self.time_unit()));
             } else {
                 return Ok(Int64Chunked::full_null(self.name(), self.len())
@@ -172,9 +169,7 @@ impl PolarsRound for DurationChunked {
                     TimeUnit::Microseconds => every_parsed.duration_us(),
                     TimeUnit::Nanoseconds => every_parsed.duration_ns(),
                 };
-                // Round half-way values away from zero
-                let half_away = t.signum() * every / 2;
-                Ok(Some(t + half_away - (t + half_away) % every))
+                Ok(Some(simple_round(t, every)))
             },
             _ => Ok(None),
         });
diff --git a/crates/polars-time/src/truncate.rs b/crates/polars-time/src/truncate.rs
index b7380378e1cd..25d6c3f2a206 100644
--- a/crates/polars-time/src/truncate.rs
+++ b/crates/polars-time/src/truncate.rs
@@ -12,6 +12,11 @@ pub trait PolarsTruncate {
         Self: Sized;
 }
 
+fn simple_truncate(t: i64, every: i64) -> i64 {
+    let remainder = t % every;
+    t - (remainder + every * (remainder < 0) as i64)
+}
+
 impl PolarsTruncate for DatetimeChunked {
     fn truncate(&self, tz: Option<&Tz>, every: &StringChunked) -> PolarsResult<Self> {
         let time_zone = self.time_zone();
@@ -33,10 +38,7 @@ impl PolarsTruncate for DatetimeChunked {
                         TimeUnit::Nanoseconds => every_parsed.duration_ns(),
                     };
                     return Ok(self
-                        .apply_values(|t| {
-                            let remainder = t % every;
-                            t - (remainder + every * (remainder < 0) as i64)
-                        })
+                        .apply_values(|t| simple_truncate(t, every))
                         .into_datetime(self.time_unit(), time_zone.clone()));
                 } else {
                     let w = Window::new(every_parsed, every_parsed, offset);
@@ -140,7 +142,7 @@ impl PolarsTruncate for DurationChunked {
                     TimeUnit::Nanoseconds => every_parsed.duration_ns(),
                 };
                 return Ok(self
-                    .apply_values(|t| t - t % every)
+                    .apply_values(|t: i64| simple_truncate(t, every))
                     .into_duration(self.time_unit()));
             } else {
                 return Ok(Int64Chunked::full_null(self.name(), self.len())
@@ -165,7 +167,7 @@ impl PolarsTruncate for DurationChunked {
                     TimeUnit::Microseconds => every_parsed.duration_us(),
                     TimeUnit::Nanoseconds => every_parsed.duration_ns(),
                 };
-                Ok(Some(t - t % every))
+                Ok(Some(simple_truncate(t, every)))
             },
             _ => Ok(None),
         });
diff --git a/py-polars/tests/unit/operations/namespaces/temporal/test_truncate.py b/py-polars/tests/unit/operations/namespaces/temporal/test_truncate.py
index 7a7fee428d5b..868422ac65e4 100644
--- a/py-polars/tests/unit/operations/namespaces/temporal/test_truncate.py
+++ b/py-polars/tests/unit/operations/namespaces/temporal/test_truncate.py
@@ -174,7 +174,7 @@ def test_truncate_duration(time_unit: TimeUnit) -> None:
             timedelta(seconds=30),
             timedelta(seconds=50),
             None,
-            timedelta(seconds=-30),
+            timedelta(seconds=-40),
         ]
     ).dt.cast_time_unit(time_unit)