
**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
dags = collect_dags(
            [
                "airflow/providers/*/example_dags",
                "airflow/providers/*/*/example_dags",
            ]
        )
        
        for dag in dags.values():
            serialized_dag = SerializedDAG.from_json(SerializedDAG.to_json(dag))
            self.validate_deserialized_dag(serialized_dag, dag)
    def validate_deserialized_dag(self, serialized_dag, dag):