
**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
"""A serialized DAG can be deserialized in another process."""
        
        
        
        queue = multiprocessing.Queue()
        proc = multiprocessing.Process(target=serialize_subprocess, args=(queue, "airflow/example_dags"))
        proc.daemon = True
        proc.start()
        stringified_dags = {}
        while True:
            v = queue.get()
            if v is None:
                break
            dag = SerializedDAG.from_json(v)
            assert isinstance(dag, DAG)
            stringified_dags[dag.dag_id] = dag
        dags = collect_dags("airflow/example_dags")
        assert set(stringified_dags.keys()) == set(dags.keys())
        
        for dag_id in stringified_dags:
            self.validate_deserialized_dag(stringified_dags[dag_id], dags[dag_id])
    def test_roundtrip_provider_example_dags(self):