diff --git a/src/obelisk/asynchronous/core.py b/src/obelisk/asynchronous/core.py
index e9231f7..0b5d7e5 100644
--- a/src/obelisk/asynchronous/core.py
+++ b/src/obelisk/asynchronous/core.py
@@ -42,7 +42,9 @@
 from obelisk.types.core import IngestMode
 
 
-DataType = Literal["number", "number[]", "json", "bool", "string", "integer", "integer[]"]
+DataType = Literal[
+    "number", "number[]", "json", "bool", "string", "integer", "integer[]"
+]
 """The possible types of data Obelisk can accept"""
 
 
@@ -168,7 +170,12 @@ class QueryParams(BaseModel):
     """List of Field Names, with their potential prefixes and suffixes, to select ordering. None user server defaults."""
     dataType: DataType | None = None
     """Data type expected to be returned, is mandatory if the `value` field is requested in the `fields` parameter"""
-    filter_: Annotated[str | Filter | None, Field(serialization_alias="filter",)] = None
+    filter_: Annotated[
+        str | Filter | None,
+        Field(
+            serialization_alias="filter",
+        ),
+    ] = None
     """
     Obelisk CORE handles filtering in [RSQL format](https://obelisk.pages.ilabt.imec.be/obelisk-core/query.html#rsql-format),
     to make it easier to also programatically write these filters, we provide the `obelisk.types.core.Filter` option as well.
@@ -192,7 +199,7 @@ def to_dict(self) -> dict[str, Any]:
             exclude_none=True, by_alias=True, mode="json", exclude={"dataset"}
         )
 
-    @field_serializer('filter_', mode='plain')
+    @field_serializer("filter_", mode="plain")
     def serialize_filter(self, value: Filter | str | None) -> str | None:
         if value is None or isinstance(value, str):
             return value
@@ -323,7 +330,7 @@ async def send(
         response = await self.http_post(
             f"{self.kind.root_url}/{dataset}/data/ingest",
             data=[x.model_dump(mode="json") for x in data],
-            params={"mode": ingest_mode.value}
+            params={"mode": ingest_mode.value},
         )
         if response.status_code != 204:
             msg = f"An error occured during data ingest. Status {response.status_code}, message: {response.text}"
diff --git a/src/obelisk/types/core.py b/src/obelisk/types/core.py
index bf460e0..2e6df8d 100644
--- a/src/obelisk/types/core.py
+++ b/src/obelisk/types/core.py
@@ -4,20 +4,21 @@
 To create a filter, look at `Filter`.
 Example:
 
->>> from datetime import datetime
+>>> from datetime import datetime, timezone
 >>> f = (Filter().add_and(
 ...     Comparison.equal('source', 'test source'),
 ...     Comparison.is_in('metricType', ['number', 'number[]']),
 ... ).add_or(
-...     Comparison.less('timestamp', datetime.fromtimestamp(1757422128))
+...     Comparison.less('timestamp', datetime.fromtimestamp(1757422128).astimezone(timezone.utc))
 ... ))
 >>> print(f)
-(('source'=='test source';'metricType'=in=('number', 'number[]')),'timestamp'<'2025-09-09T14:48:48')
+(('source'=='test source';'metricType'=in=('number', 'number[]')),'timestamp'<'2025-09-09T12:48:48+00:00')
 """
 
 from __future__ import annotations
 from abc import ABC
 from datetime import datetime
+
 from typing import Any
 from collections.abc import Iterable
 from enum import Enum
@@ -55,6 +56,8 @@ class Comparison:
     When serializing to RSQL format,
     each argument is single quoted as to accept any otherwise reserved characters,
     and serialised using :func:`str`.
+
+    If the right-hand value is a datetime, it _must_ be timezone-aware.
     """
 
     left: FieldName
@@ -77,6 +80,8 @@ def __str__(self) -> str:
     def _sstr(item: Any) -> str:
         """Smart string conversion"""
         if isinstance(item, datetime):
+            if item.tzinfo is None:
+                raise ValueError(f"Right side of comparison is naive datetime: {item}")
             return item.isoformat()
         return str(item)
 
@@ -204,6 +209,7 @@ def add_or(self, *other: Item) -> Filter:
 
 class IngestMode(str, Enum):
     """Whether the ingested datapoints should be streamed, stored, or both (the default)"""
+
     BOTH = "DEFAULT"
     STREAM = "STREAM_ONLY"
     STORE = "STORE_ONLY"
diff --git a/src/tests/typetest/filter_test.py b/src/tests/typetest/filter_test.py
index 9d547e3..96446ef 100644
--- a/src/tests/typetest/filter_test.py
+++ b/src/tests/typetest/filter_test.py
@@ -1,9 +1,10 @@
 from obelisk.types.core import Filter, Comparison
-from datetime import datetime
+from datetime import datetime, timezone
+import pytest
 
 
 def test_basic_filter():
-    test_dt = datetime.now()
+    test_dt = datetime.now(tz=timezone.utc)
     f = (
         Filter()
         .add_and(
@@ -17,3 +18,9 @@ def test_basic_filter():
 
     expected = f"((('source'=='test source'),'timestamp'<'{test_dt.isoformat()}'),'metricType'=in=('number', 'number[]'))"
     assert str(f) == expected
+
+
+def test_naive_datetime():
+    with pytest.raises(ValueError):
+        c = Comparison.less("timestamp", datetime.fromtimestamp(1879282))
+        print(str(c))