
**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
"""
        Test that params work both on Serialized DAGs & Tasks
        """
        dag = DAG(dag_id='simple_dag', params=val)
        BaseOperator(task_id='simple_task', dag=dag, start_date=datetime(2019, 8, 1))
        serialized_dag = SerializedDAG.to_dict(dag)
        if val:
            assert "params" in serialized_dag["dag"]
        else:
            assert "params" not in serialized_dag["dag"]
        deserialized_dag = SerializedDAG.from_dict(serialized_dag)
        deserialized_simple_task = deserialized_dag.task_dict["simple_task"]
        assert expected_val == deserialized_dag.params
        assert expected_val == deserialized_simple_task.params
    @parameterized.expand(
        [
            (None, {}),
            ({"param_1": "value_1"}, {"param_1": "value_1"}),
        ]
    )
    def test_task_params_roundtrip(self, val, expected_val):