fix: treat SQLite database update time as UTC for session's last update time

Fixes https://github.com/google/adk-python/issues/1180

We are using `func.now()` to set the `onupdate` time for db, when SQLAlchemy generates the SQL to build the database, it actually translates `func.now()` into `NOW()` or `CURRENT_TIMESTAMP`. The value it returns depends on the database server settings. For example, if the global/default timezone for a db is set to be UTC, the update time will be set to be a UCT time; if the global time zone for a db is set to be a local time zone (e.g. America/Los_Angeles), the update time will be a local time.

Normally, the best practice is to set database server to use UTC. Applications will convert it into different time zones as needed.

For SQLite, there is no way to config the default timezone, it will just treat it as UTC. But because it is a naive datetime (with no timezone info), python will assume it is a local time and then covert it into a UTC, which is why we see the bug (e.g. we create a session at 2025-06-17 12:49:33 local time, but when we read the session, its last update time is 2025-06-17 19:49:33 local time).

The solution is converting the native datatime to be timezone aware before `.timestamp()`.

The change in this CL only affects SQLite database.

PiperOrigin-RevId: 776654443

src/google/adk/sessions/database_session_service.py

@@ -15,6 +15,7 @@ from __future__ import annotations
 
 import copy
 from datetime import datetime
+from datetime import timezone
 import json
 import logging
 from typing import Any
@@ -144,6 +145,21 @@ class StorageSession(Base):
   def __repr__(self):
     return f"<StorageSession(id={self.id}, update_time={self.update_time})>"
 
+  @property
+  def _dialect_name(self) -> Optional[str]:
+    session = inspect(self).session
+    return session.bind.dialect.name if session else None
+
+  @property
+  def update_timestamp_tz(self) -> datetime:
+    """Returns the time zone aware update timestamp."""
+    if self._dialect_name == "sqlite":
+      # SQLite does not support timezone. SQLAlchemy returns a naive datetime
+      # object without timezone information. We need to convert it to UTC
+      # manually.
+      return self.update_time.replace(tzinfo=timezone.utc).timestamp()
+    return self.update_time.timestamp()
+
 
 class StorageEvent(Base):
   """Represents an event stored in the database."""
@@ -412,7 +428,7 @@ class DatabaseSessionService(BaseSessionService):
           user_id=str(storage_session.user_id),
           id=str(storage_session.id),
           state=merged_state,
-          last_update_time=storage_session.update_time.timestamp(),
+          last_update_time=storage_session.update_timestamp_tz,
       )
       return session
 
@@ -473,7 +489,7 @@ class DatabaseSessionService(BaseSessionService):
           user_id=user_id,
           id=session_id,
           state=merged_state,
-          last_update_time=storage_session.update_time.timestamp(),
+          last_update_time=storage_session.update_timestamp_tz,
       )
       session.events = [e.to_event() for e in reversed(storage_events)]
     return session
@@ -496,7 +512,7 @@ class DatabaseSessionService(BaseSessionService):
             user_id=user_id,
             id=storage_session.id,
             state={},
-            last_update_time=storage_session.update_time.timestamp(),
+            last_update_time=storage_session.update_timestamp_tz,
         )
         sessions.append(session)
       return ListSessionsResponse(sessions=sessions)
@@ -529,13 +545,13 @@ class DatabaseSessionService(BaseSessionService):
           StorageSession, (session.app_name, session.user_id, session.id)
       )
 
-      if storage_session.update_time.timestamp() > session.last_update_time:
+      if storage_session.update_timestamp_tz > session.last_update_time:
         raise ValueError(
             "The last_update_time provided in the session object"
             f" {datetime.fromtimestamp(session.last_update_time):'%Y-%m-%d %H:%M:%S'} is"
             " earlier than the update_time in the storage_session"
-            f" {storage_session.update_time:'%Y-%m-%d %H:%M:%S'}. Please check"
-            " if it is a stale session."
+            f" {datetime.fromtimestamp(storage_session.update_timestamp_tz):'%Y-%m-%d %H:%M:%S'}."
+            " Please check if it is a stale session."
         )
 
       # Fetch states from storage
@@ -577,7 +593,7 @@ class DatabaseSessionService(BaseSessionService):
       session_factory.refresh(storage_session)
 
       # Update timestamp with commit time
-      session.last_update_time = storage_session.update_time.timestamp()
+      session.last_update_time = storage_session.update_timestamp_tz
 
     # Also update the in-memory session
     await super().append_event(session=session, event=event)

tests/unittests/sessions/test_session_service.py

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from datetime import datetime
+from datetime import timezone
 import enum
 
 from google.adk.events import Event
@@ -66,10 +68,17 @@ async def test_create_get_session(service_type):
   assert session.id
   assert session.state == state
   assert (
-      await session_service.get_session(
-          app_name=app_name, user_id=user_id, session_id=session.id
-      )
-      == session
+      session.last_update_time
+      <= datetime.now().astimezone(timezone.utc).timestamp()
+  )
+
+  got_session = await session_service.get_session(
+      app_name=app_name, user_id=user_id, session_id=session.id
+  )
+  assert got_session == session
+  assert (
+      got_session.last_update_time
+      <= datetime.now().astimezone(timezone.utc).timestamp()
   )
 
   session_id = session.id