diff --git a/src/ni/datastore/data/_grpc_conversion.py b/src/ni/datastore/data/_grpc_conversion.py
index c87fbd8..5dd6c14 100644
--- a/src/ni/datastore/data/_grpc_conversion.py
+++ b/src/ni/datastore/data/_grpc_conversion.py
@@ -58,6 +58,11 @@
 _logger = logging.getLogger(__name__)
 
 
+def _ensure_utc(dt: ht.datetime) -> ht.datetime:
+    """Convert a datetime to UTC, treating naive datetimes as local time."""
+    return dt.astimezone(std_datetime.timezone.utc)
+
+
 def populate_publish_condition_request_value(
     publish_request: PublishConditionRequest, value: object
 ) -> None:
diff --git a/src/ni/datastore/data/_types/_step.py b/src/ni/datastore/data/_types/_step.py
index 4bc598f..b2da8de 100644
--- a/src/ni/datastore/data/_types/_step.py
+++ b/src/ni/datastore/data/_types/_step.py
@@ -5,6 +5,7 @@
 from typing import Mapping, MutableMapping
 
 import hightime as ht
+from ni.datastore.data._grpc_conversion import _ensure_utc
 from ni.datastore.data._types._error_information import ErrorInformation
 from ni.datastore.data._types._outcome import Outcome
 from ni.datastore.metadata._grpc_conversion import (
@@ -147,12 +148,14 @@ def to_protobuf(self) -> StepProto:
             step_type=self.step_type,
             notes=self.notes,
             start_date_time=(
-                hightime_datetime_to_protobuf(self.start_date_time)
+                hightime_datetime_to_protobuf(_ensure_utc(self.start_date_time))
                 if self.start_date_time
                 else None
             ),
             end_date_time=(
-                hightime_datetime_to_protobuf(self.end_date_time) if self.end_date_time else None
+                hightime_datetime_to_protobuf(_ensure_utc(self.end_date_time))
+                if self.end_date_time
+                else None
             ),
             link=self.link,
             schema_id=self.schema_id,
diff --git a/src/ni/datastore/data/_types/_test_result.py b/src/ni/datastore/data/_types/_test_result.py
index 6bc9cda..6a41eef 100644
--- a/src/ni/datastore/data/_types/_test_result.py
+++ b/src/ni/datastore/data/_types/_test_result.py
@@ -5,6 +5,7 @@
 from typing import Iterable, Mapping, MutableMapping, MutableSequence
 
 import hightime as ht
+from ni.datastore.data._grpc_conversion import _ensure_utc
 from ni.datastore.data._types._error_information import ErrorInformation
 from ni.datastore.data._types._outcome import Outcome
 from ni.datastore.metadata._grpc_conversion import (
@@ -182,12 +183,14 @@ def to_protobuf(self) -> TestResultProto:
             test_adapter_ids=self.test_adapter_ids,
             name=self.name,
             start_date_time=(
-                hightime_datetime_to_protobuf(self.start_date_time)
+                hightime_datetime_to_protobuf(_ensure_utc(self.start_date_time))
                 if self.start_date_time
                 else None
             ),
             end_date_time=(
-                hightime_datetime_to_protobuf(self.end_date_time) if self.end_date_time else None
+                hightime_datetime_to_protobuf(_ensure_utc(self.end_date_time))
+                if self.end_date_time
+                else None
             ),
             outcome=self.outcome.to_protobuf(),
             link=self.link,
diff --git a/tests/unit/data/test_create_metadata.py b/tests/unit/data/test_create_metadata.py
index 04b5112..c29dde2 100644
--- a/tests/unit/data/test_create_metadata.py
+++ b/tests/unit/data/test_create_metadata.py
@@ -2,20 +2,24 @@
 
 from __future__ import annotations
 
+import datetime as std_datetime
 from typing import cast
 from unittest.mock import NonCallableMock
 
+import hightime as ht
 from ni.datastore.data import (
     DataStoreClient,
     Step,
     TestResult,
 )
+from ni.datastore.data._types._outcome import Outcome
 from ni.measurements.data.v1.data_store_service_pb2 import (
     CreateStepRequest,
     CreateStepResponse,
     CreateTestResultRequest,
     CreateTestResultResponse,
 )
+from ni.protobuf.types.precision_timestamp_conversion import hightime_datetime_to_protobuf
 
 
 def test___create_step___calls_data_store_service_client(
@@ -66,3 +70,77 @@ def test___create_test_result___calls_data_store_service_client(
     request = cast(CreateTestResultRequest, args[0])
     assert request.test_result == test_result.to_protobuf()
     assert result == "response_id"
+
+
+def test___test_result_to_protobuf___naive_datetime___converts_to_utc() -> None:
+    naive_start = ht.datetime(2024, 6, 15, 10, 30, 0)
+    naive_end = ht.datetime(2024, 6, 15, 11, 0, 0)
+    test_result = TestResult(
+        name="test",
+        start_date_time=naive_start,
+        end_date_time=naive_end,
+        outcome=Outcome.PASSED,
+    )
+
+    proto = test_result.to_protobuf()
+
+    expected_start = hightime_datetime_to_protobuf(
+        naive_start.astimezone(std_datetime.timezone.utc)
+    )
+    expected_end = hightime_datetime_to_protobuf(naive_end.astimezone(std_datetime.timezone.utc))
+    assert proto.start_date_time == expected_start
+    assert proto.end_date_time == expected_end
+
+
+def test___test_result_to_protobuf___utc_datetime___preserved() -> None:
+    utc_start = ht.datetime(2024, 6, 15, 10, 30, 0, tzinfo=std_datetime.timezone.utc)
+    utc_end = ht.datetime(2024, 6, 15, 11, 0, 0, tzinfo=std_datetime.timezone.utc)
+    test_result = TestResult(
+        name="test",
+        start_date_time=utc_start,
+        end_date_time=utc_end,
+        outcome=Outcome.PASSED,
+    )
+
+    proto = test_result.to_protobuf()
+
+    expected_start = hightime_datetime_to_protobuf(utc_start)
+    expected_end = hightime_datetime_to_protobuf(utc_end)
+    assert proto.start_date_time == expected_start
+    assert proto.end_date_time == expected_end
+
+
+def test___step_to_protobuf___naive_datetime___converts_to_utc() -> None:
+    naive_start = ht.datetime(2024, 6, 15, 10, 30, 0)
+    naive_end = ht.datetime(2024, 6, 15, 11, 0, 0)
+    step = Step(
+        name="step",
+        start_date_time=naive_start,
+        end_date_time=naive_end,
+    )
+
+    proto = step.to_protobuf()
+
+    expected_start = hightime_datetime_to_protobuf(
+        naive_start.astimezone(std_datetime.timezone.utc)
+    )
+    expected_end = hightime_datetime_to_protobuf(naive_end.astimezone(std_datetime.timezone.utc))
+    assert proto.start_date_time == expected_start
+    assert proto.end_date_time == expected_end
+
+
+def test___step_to_protobuf___utc_datetime___preserved() -> None:
+    utc_start = ht.datetime(2024, 6, 15, 10, 30, 0, tzinfo=std_datetime.timezone.utc)
+    utc_end = ht.datetime(2024, 6, 15, 11, 0, 0, tzinfo=std_datetime.timezone.utc)
+    step = Step(
+        name="step",
+        start_date_time=utc_start,
+        end_date_time=utc_end,
+    )
+
+    proto = step.to_protobuf()
+
+    expected_start = hightime_datetime_to_protobuf(utc_start)
+    expected_end = hightime_datetime_to_protobuf(utc_end)
+    assert proto.start_date_time == expected_start
+    assert proto.end_date_time == expected_end