
**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
"""
        This test verifies that there are no new fields added to BaseOperator. And reminds that
        tests should be added for it.
        """
        base_operator = BaseOperator(task_id="10")
        fields = base_operator.__dict__
        assert {
            '_BaseOperator__instantiated': True,
            '_dag': None,
            '_downstream_task_ids': set(),
            '_inlets': [],
            '_log': base_operator.log,
            '_outlets': [],
            '_upstream_task_ids': set(),
            'depends_on_past': False,
            'do_xcom_push': True,
            'email': None,
            'email_on_failure': True,
            'email_on_retry': True,
            'end_date': None,
            'execution_timeout': None,
            'executor_config': {},
            'inlets': [],
            'label': '10',
            'max_retry_delay': None,
            'on_execute_callback': None,
            'on_failure_callback': None,
            'on_retry_callback': None,
            'on_success_callback': None,