
**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 extra field exists & OperatorLinks defined in Plugins and inbuilt Operator Links.
        This tests also depends on GoogleLink() registered as a plugin
        in tests/plugins/test_plugin.py
        The function tests that if extra operator links are registered in plugin
        in ``operator_extra_links`` and the same is also defined in
        the Operator in ``BaseOperator.operator_extra_links``, it has the correct
        extra link.
        """
        test_date = datetime(2019, 8, 1)
        dag = DAG(dag_id='simple_dag', start_date=test_date)
        CustomOperator(task_id='simple_task', dag=dag, bash_command=["echo", "true"])
        serialized_dag = SerializedDAG.to_dict(dag)
        assert "bash_command" in serialized_dag["dag"]["tasks"][0]
        dag = SerializedDAG.from_dict(serialized_dag)
        simple_task = dag.task_dict["simple_task"]
        assert getattr(simple_task, "bash_command") == ["echo", "true"]
        
        
        
        
        assert serialized_dag["dag"]["tasks"][0]["_operator_extra_links"] == [
            {'tests.test_utils.mock_operators.CustomBaseIndexOpLink': {'index': 0}},
            {'tests.test_utils.mock_operators.CustomBaseIndexOpLink': {'index': 1}},
        ]