
**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
"""Make very simple DAG to verify serialization result."""
    with DAG(
        dag_id='simple_dag',
        default_args={
            "retries": 1,
            "retry_delay": timedelta(minutes=5),
            "depends_on_past": False,
            "sla": timedelta(seconds=100),
        },
        start_date=datetime(2019, 8, 1),
        is_paused_upon_creation=False,
        access_control={"test_role": {permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT}},
    ) as dag:
        CustomOperator(task_id='custom_task')
        BashOperator(
            task_id='bash_task',
            bash_command='echo {{ task.task_id }}',
            owner='airflow',
            executor_config={"pod_override": executor_config_pod},
        )
        return {'simple_dag': dag}
def make_user_defined_macro_filter_dag():
    """Make DAGs with user defined macros and filters using locally defined methods.
    For Webserver, we do not include ``user_defined_macros`` & ``user_defined_filters``.
    The examples here test:
        (1) functions can be successfully displayed on UI;
        (2) templates with function macros have been rendered before serialization.
    """
    def compute_next_execution_date(dag, execution_date):