
**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=dag_start_date)
        BaseOperator(task_id='simple_task', dag=dag, start_date=task_start_date)
        serialized_dag = SerializedDAG.to_dict(dag)
        if not task_start_date or dag_start_date >= task_start_date:
            
            
            assert "start_date" not in serialized_dag["dag"]["tasks"][0]
        else:
            assert "start_date" in serialized_dag["dag"]["tasks"][0]
        dag = SerializedDAG.from_dict(serialized_dag)
        simple_task = dag.task_dict["simple_task"]
        assert simple_task.start_date == expected_task_start_date
    def test_deserialization_with_dag_context(self):