
**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
"""BaseSerialization provides utils for serialization."""
    
    _primitive_types = (int, bool, float, str)
    
    
    _datetime_types = (datetime.datetime,)
    
    _excluded_types = (logging.Logger, Connection, type)
    _json_schema: Optional[Validator] = None
    
    
    
    _load_operator_extra_links = True
    _CONSTRUCTOR_PARAMS: Dict[str, Parameter] = {}
    SERIALIZER_VERSION = 1
    @classmethod
    def to_json(cls, var: Union[DAG, BaseOperator, dict, list, set, tuple]) -> str: