
**Apache Airflow version**:

master

**What happened**:

https://github.com/apache/airflow/blob/master/airflow/models/baseoperator.py#L356 --> should be timedelta not datetime


tests/serialization/test_dag_serialization.py
'outlets': [],
            'owner': 'airflow',
            'params': {},
            'pool': 'default_pool',
            'pool_slots': 1,
            'priority_weight': 1,
            'queue': 'default',
            'resources': None,
            'retries': 0,
            'retry_delay': timedelta(0, 300),
            'retry_exponential_backoff': False,
            'run_as_user': None,
            'sla': None,
            'start_date': None,
            'subdag': None,
            'task_concurrency': None,
            'task_id': '10',
            'trigger_rule': 'all_success',
            'wait_for_downstream': False,
            'weight_rule': 'downstream',
        } == fields, """
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     ACTION NEEDED! PLEASE READ THIS CAREFULLY AND CORRECT TESTS CAREFULLY
 Some fields were added to the BaseOperator! Please add them to the list above and make sure that
 you add support for DAG serialization - you should add the field to
 `airflow/serialization/schema.json` - they should have correct type defined there.
 Note that we do not support versioning yet so you should only add optional fields to BaseOperator.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
                         """
    def test_task_group_serialization(self):