
**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
dag = DAG(dag_id='simple_dag', start_date=datetime(2019, 8, 1), end_date=dag_end_date)
        BaseOperator(task_id='simple_task', dag=dag, end_date=task_end_date)
        serialized_dag = SerializedDAG.to_dict(dag)
        if not task_end_date or dag_end_date <= task_end_date:
            
            
            assert "end_date" not in serialized_dag["dag"]["tasks"][0]
        else:
            assert "end_date" in serialized_dag["dag"]["tasks"][0]
        dag = SerializedDAG.from_dict(serialized_dag)
        simple_task = dag.task_dict["simple_task"]
        assert simple_task.end_date == expected_task_end_date
    @parameterized.expand(
        [
            (None, None, None),
            ("@weekly", "@weekly", "0 0 * * 0"),
            ("@once", "@once", None),
            ({"__type": "timedelta", "__var": 86400.0}, timedelta(days=1), timedelta(days=1)),
        ]
    )
    def test_deserialization_schedule_interval(