
**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
serialized = SerializedDAG._serialize(val)
        assert serialized == expected
        round_tripped = SerializedDAG._deserialize(serialized)
        assert val == round_tripped
    @parameterized.expand(
        [
            (None, {}),
            ({"param_1": "value_1"}, {"param_1": "value_1"}),
        ]
    )
    def test_dag_params_roundtrip(self, val, expected_val):