diff --git a/crates/polars-core/src/utils/mod.rs b/crates/polars-core/src/utils/mod.rs
index c85c807096f57..3be69c2cced80 100644
--- a/crates/polars-core/src/utils/mod.rs
+++ b/crates/polars-core/src/utils/mod.rs
@@ -554,8 +554,8 @@ macro_rules! df {
 pub fn get_time_units(tu_l: &TimeUnit, tu_r: &TimeUnit) -> TimeUnit {
     use TimeUnit::*;
     match (tu_l, tu_r) {
-        (Nanoseconds, Microseconds) => Microseconds,
-        (_, Milliseconds) => Milliseconds,
+        (_, Nanoseconds) => Nanoseconds,
+        (Milliseconds, Microseconds) => Microseconds,
         _ => *tu_l,
     }
 }
diff --git a/crates/polars/tests/it/core/date_like.rs b/crates/polars/tests/it/core/date_like.rs
index 8d97b5e87d1de..db4850726a22a 100644
--- a/crates/polars/tests/it/core/date_like.rs
+++ b/crates/polars/tests/it/core/date_like.rs
@@ -155,6 +155,57 @@ fn test_duration() -> PolarsResult<()> {
             .into_duration(TimeUnit::Nanoseconds)
             .into_series()
     );
+
+    Ok(())
+}
+
+#[test]
+#[cfg(feature = "dtype-duration")]
+fn test_duration_arithmetic_cross_timeunit() -> PolarsResult<()> {
+    let datetime_ns = Int64Chunked::new("", &[2, 3, 4])
+        .into_datetime(TimeUnit::Nanoseconds, None)
+        .into_series();
+
+    let datetime_us = Int64Chunked::new("", &[2, 3, 4])
+        .into_datetime(TimeUnit::Microseconds, None)
+        .into_series();
+
+    let datetime_ms = Int64Chunked::new("", &[2, 3, 4])
+        .into_datetime(TimeUnit::Milliseconds, None)
+        .into_series();
+
+    let duration_ns = Int64Chunked::new("", &[1, 1, 1])
+        .into_duration(TimeUnit::Nanoseconds)
+        .into_series();
+
+    let duration_us = Int64Chunked::new("", &[1, 1, 1])
+        .into_duration(TimeUnit::Microseconds)
+        .into_series();
+
+    let duration_ms = Int64Chunked::new("", &[1, 1, 1])
+        .into_duration(TimeUnit::Milliseconds)
+        .into_series();
+
+    assert_eq!(
+        *(datetime_ns.clone() + duration_us).dtype(),
+        DataType::Datetime(TimeUnit::Nanoseconds, None)
+    );
+
+    assert_eq!(
+        *(datetime_ns + duration_ms.clone()).dtype(),
+        DataType::Datetime(TimeUnit::Nanoseconds, None)
+    );
+
+    assert_eq!(
+        *(datetime_us + duration_ms).dtype(),
+        DataType::Datetime(TimeUnit::Microseconds, None)
+    );
+
+    assert_eq!(
+        *(datetime_ms + duration_ns).dtype(),
+        DataType::Datetime(TimeUnit::Nanoseconds, None)
+    );
+
     Ok(())
 }
 
diff --git a/py-polars/tests/unit/functions/as_datatype/test_duration.py b/py-polars/tests/unit/functions/as_datatype/test_duration.py
index 07dc165a8a0fb..bb8f1afc443ed 100644
--- a/py-polars/tests/unit/functions/as_datatype/test_duration.py
+++ b/py-polars/tests/unit/functions/as_datatype/test_duration.py
@@ -143,6 +143,49 @@ def test_add_duration_3786() -> None:
     }
 
 
+def test_datetime_ms_add_duration_us() -> None:
+    duration_us = pl.duration(microseconds=15)
+    df_datetimes = pl.DataFrame(
+        {
+            "datetime": [
+                datetime(2000, 1, 1, 1, second=30),
+                datetime(2000, 1, 1, 1, second=35),
+                datetime(2000, 1, 1, 1, second=40),
+            ]
+        },
+        schema={"datetime": pl.Datetime(time_unit="ms")},
+    )
+
+    df_add_us = df_datetimes.select(pl.col("datetime") + duration_us)
+
+    assert df_add_us["datetime"].dt.microsecond().to_list() == [15, 15, 15]
+
+    assert df_add_us["datetime"].dtype.time_unit == "us"
+
+
+def test_datetime_us_add_duration_ns() -> None:
+    duration_ns = pl.duration(nanoseconds=15, time_unit="ns")
+    df_datetimes = pl.DataFrame(
+        {
+            "datetime": [
+                datetime(2000, 1, 1, 1, microsecond=30),
+                datetime(2000, 1, 1, 1, microsecond=35),
+                datetime(2000, 1, 1, 1, microsecond=40),
+            ]
+        }
+    )
+
+    df_add_ns = df_datetimes.select(pl.col("datetime") + duration_ns)
+
+    assert df_add_ns["datetime"].dt.nanosecond().to_list() == [
+        15 + 30 * 1000,
+        15 + 35 * 1000,
+        15 + 40 * 1000,
+    ]
+
+    assert df_add_ns["datetime"].dtype.time_unit == "ns"
+
+
 @pytest.mark.parametrize(
     ("time_unit", "ms", "us", "ns"),
     [