diff --git a/backend/routers/project_locations.py b/backend/routers/project_locations.py
index 02003d6..7059192 100644
--- a/backend/routers/project_locations.py
+++ b/backend/routers/project_locations.py
@@ -874,8 +874,8 @@ async def delete_assignment(
         and_(
             MonitoringSession.location_id == assignment.location_id,
             MonitoringSession.unit_id == assignment.unit_id,
-            MonitoringSession.start_time >= window_start,
-            MonitoringSession.start_time <= window_end,
+            MonitoringSession.started_at >= window_start,
+            MonitoringSession.started_at <= window_end,
         )
     ).count()