
**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
assert getattr(serialized_task, field) == getattr(
                task, field
            ), f'{task.dag.dag_id}.{task.task_id}.{field} does not match'
        if serialized_task.resources is None:
            assert task.resources is None or task.resources == []
        else:
            assert serialized_task.resources == task.resources
        
        
        if task.task_type == "SubDagOperator":
            assert serialized_task.subdag is not None
            assert isinstance(serialized_task.subdag, DAG)
        else:
            assert serialized_task.subdag is None
    @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, 2, tzinfo=timezone.utc),
            ),
            (
                datetime(2019, 8, 1, tzinfo=timezone.utc),
                datetime(2019, 7, 30, tzinfo=timezone.utc),
                datetime(2019, 8, 1, tzinfo=timezone.utc),
            ),
        ]
    )
    def test_deserialization_start_date(self, dag_start_date, task_start_date, expected_task_start_date):