diff --git a/app/assets/api/schemas_out.py b/app/assets/api/schemas_out.py
index f36447856..db7efbd4b 100644
--- a/app/assets/api/schemas_out.py
+++ b/app/assets/api/schemas_out.py
@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any
 
 from pydantic import BaseModel, ConfigDict, Field, field_serializer
@@ -20,7 +20,11 @@ class AssetSummary(BaseModel):
 
     @field_serializer("created_at", "updated_at", "last_access_time")
     def _serialize_datetime(self, v: datetime | None, _info):
-        return v.isoformat() if v else None
+        if v is None:
+            return None
+        if v.tzinfo is None:
+            v = v.replace(tzinfo=timezone.utc)
+        return v.isoformat()
 
 
 class AssetsList(BaseModel):
@@ -41,7 +45,11 @@ class AssetUpdated(BaseModel):
 
     @field_serializer("updated_at")
     def _serialize_updated_at(self, v: datetime | None, _info):
-        return v.isoformat() if v else None
+        if v is None:
+            return None
+        if v.tzinfo is None:
+            v = v.replace(tzinfo=timezone.utc)
+        return v.isoformat()
 
 
 class AssetDetail(BaseModel):
@@ -60,7 +68,11 @@ class AssetDetail(BaseModel):
 
     @field_serializer("created_at", "last_access_time")
     def _serialize_datetime(self, v: datetime | None, _info):
-        return v.isoformat() if v else None
+        if v is None:
+            return None
+        if v.tzinfo is None:
+            v = v.replace(tzinfo=timezone.utc)
+        return v.isoformat()
 
 
 class AssetCreated(AssetDetail):
diff --git a/app/assets/helpers.py b/app/assets/helpers.py
index 3798f3933..4c2b0f681 100644
--- a/app/assets/helpers.py
+++ b/app/assets/helpers.py
@@ -34,7 +34,7 @@ def escape_sql_like_string(s: str, escape: str = "!") -> tuple[str, str]:
 
 
 def get_utc_now() -> datetime:
-    """Naive UTC timestamp (no tzinfo). We always treat DB datetimes as UTC."""
+    """Naive UTC timestamp (no tzinfo) for DB columns declared with timezone=False."""
     return datetime.now(timezone.utc).replace(tzinfo=None)
 
 
diff --git a/app/database/models.py b/app/database/models.py
index e7572677a..aefcfdcf2 100644
--- a/app/database/models.py
+++ b/app/database/models.py
@@ -1,5 +1,5 @@
 from typing import Any
-from datetime import datetime
+from datetime import datetime, timezone
 from sqlalchemy.orm import DeclarativeBase
 
 class Base(DeclarativeBase):
@@ -13,6 +13,8 @@ def to_dict(obj: Any, include_none: bool = False) -> dict[str, Any]:
         if val is None and not include_none:
             continue
         if isinstance(val, datetime):
+            if val.tzinfo is None:
+                val = val.replace(tzinfo=timezone.utc)
             out[field] = val.isoformat()
         else:
             out[field] = val