diff --git a/dashboard/backend/db/opc_db.py b/dashboard/backend/db/opc_db.py
index 147a222..f74423f 100644
--- a/dashboard/backend/db/opc_db.py
+++ b/dashboard/backend/db/opc_db.py
@@ -365,11 +365,20 @@ async def update_task(
         task = dict(row)
         task["tags"] = json.loads(task["tags"]) if task["tags"] else []
 
+        # Convert datetime objects to ISO format strings for JSON serialization
+        old_values_for_history = old_values.copy()
+        updates_for_history = updates.copy()
+        for key in ["created_at", "updated_at", "completed_at", "due_date"]:
+            if old_values_for_history.get(key):
+                old_values_for_history[key] = old_values_for_history[key].isoformat()
+            if updates_for_history.get(key):
+                updates_for_history[key] = updates_for_history[key].isoformat()
+
         # Log history
         await conn.execute("""
             INSERT INTO task_history (task_id, action, actor, actor_type, old_value, new_value)
             VALUES ($1, $2, $3, $4, $5, $6)
-        """, task_id, "updated", actor, "human", json.dumps(old_values), json.dumps(updates))
+        """, task_id, "updated", actor, "human", json.dumps(old_values_for_history), json.dumps(updates_for_history))
 
         return task
 
diff --git a/dashboard/backend/routers/opc_tasks.py b/dashboard/backend/routers/opc_tasks.py
index f099ce2..fa7f06b 100644
--- a/dashboard/backend/routers/opc_tasks.py
+++ b/dashboard/backend/routers/opc_tasks.py
@@ -221,7 +221,7 @@ async def move_task(
         await opc_db.stop_time_entry(task_id=task_id)
 
     if move.status == "done":
-        updates["completed_at"] = datetime.utcnow()
+        updates["completed_at"] = datetime.utcnow().replace(tzinfo=None)
 
     updated_task = await opc_db.update_task(
         task_id=task_id,