diff --git a/app/s3/s3_resource.py b/app/s3/s3_resource.py
index 3ef709c59a4ae45a37dc4c824366b5dda6c1a621..5f02f913e12f0ff0b96d5803a3d0c6c18a729527 100644
--- a/app/s3/s3_resource.py
+++ b/app/s3/s3_resource.py
@@ -50,5 +50,7 @@ async def get_s3_bucket_object(s3: S3ServiceResource, bucket_name: str, key: str
             await obj.load()
         except ClientError:
             return None
-        span.set_attributes({"size": await obj.size, "last_modified": await obj.last_modified, "etag": await obj.e_tag})
+        span.set_attributes(
+            {"size": await obj.size, "last_modified": (await obj.last_modified).isoformat(), "etag": await obj.e_tag}
+        )
         return obj