
**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
with DAG(dag_id='simple_dag', start_date=datetime(2019, 8, 1, tzinfo=timezone.utc)) as dag:
            BaseOperator(task_id='simple_task')
            
            SerializedDAG.to_dict(dag)
    @parameterized.expand(
        [
            (datetime(2019, 8, 1, tzinfo=timezone.utc), None, datetime(2019, 8, 1, tzinfo=timezone.utc)),
            (
                datetime(2019, 8, 1, tzinfo=timezone.utc),
                datetime(2019, 8, 2, tzinfo=timezone.utc),
                datetime(2019, 8, 1, tzinfo=timezone.utc),
            ),
            (
                datetime(2019, 8, 1, tzinfo=timezone.utc),
                datetime(2019, 7, 30, tzinfo=timezone.utc),
                datetime(2019, 7, 30, tzinfo=timezone.utc),
            ),
        ]
    )
    def test_deserialization_end_date(self, dag_end_date, task_end_date, expected_task_end_date):