diff --git a/openagent_eval/corpus/staleness.py b/openagent_eval/corpus/staleness.py
index a2703d1..5ebca17 100644
--- a/openagent_eval/corpus/staleness.py
+++ b/openagent_eval/corpus/staleness.py
@@ -7,7 +7,7 @@
 from __future__ import annotations
 
 import re
-from datetime import datetime, timedelta, timezone
+from datetime import UTC, datetime, timedelta, timezone
 from typing import Any
 
 from openagent_eval.corpus.base import BaseCorpusAnalyzer
@@ -172,6 +172,8 @@ def _extract_timestamp_from_metadata(
                 continue
 
             if isinstance(value, datetime):
+                if value.tzinfo is None:
+                    return value.replace(tzinfo=UTC)
                 return value
 
             if isinstance(value, str):
diff --git a/tests/unit/test_corpus/test_staleness.py b/tests/unit/test_corpus/test_staleness.py
index 1369207..374b45c 100644
--- a/tests/unit/test_corpus/test_staleness.py
+++ b/tests/unit/test_corpus/test_staleness.py
@@ -59,6 +59,20 @@ async def test_detects_stale_documents(self, detector, stale_doc):
         )
         assert "stale.txt" in report.issues[0].document_ids
 
+    @pytest.mark.asyncio
+    async def test_naive_metadata_datetime_is_treated_as_utc(self, detector):
+        """Test that naive metadata datetimes do not fail UTC comparisons."""
+        doc = CorpusDocument(
+            doc_id="naive.txt",
+            content="Old content.",
+            metadata={"last_modified": datetime(2020, 1, 15)},
+        )
+
+        report = await detector.analyze([doc])
+
+        assert len(report.issues) == 1
+        assert report.issues[0].metadata["last_updated"].endswith("+00:00")
+
     @pytest.mark.asyncio
     async def test_health_score_decreases_with_staleness(self, detector, recent_doc, stale_doc):
         """Test health score decreases with more stale documents."""