
**Apache Airflow version**:

master

**What happened**:

https://github.com/apache/airflow/blob/master/airflow/models/baseoperator.py#L356 --> should be timedelta not datetime


airflow/serialization/serialized_objects.py
"""Stringifies DAGs and operators contained by var and returns a JSON string of var."""
        return json.dumps(cls.to_dict(var), ensure_ascii=True)
    @classmethod
    def to_dict(cls, var: Union[DAG, BaseOperator, dict, list, set, tuple]) -> dict:
        """Stringifies DAGs and operators contained by var and returns a dict of var."""
        
        
        raise NotImplementedError()
    @classmethod
    def from_json(cls, serialized_obj: str) -> Union['BaseSerialization', dict, list, set, tuple]:
        """Deserializes json_str and reconstructs all DAGs and operators it contains."""
        return cls.from_dict(json.loads(serialized_obj))
    @classmethod
    def from_dict(
        cls, serialized_obj: Dict[Encoding, Any]
    ) -> Union['BaseSerialization', dict, list, set, tuple]:
        """Deserializes a python dict stored with type decorators and
        reconstructs all DAGs and operators it contains.
        """
        return cls._deserialize(serialized_obj)
    @classmethod
    def validate_schema(cls, serialized_obj: Union[str, dict]) -> None: