
**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
cls, object_to_serialize: Union[BaseOperator, DAG], decorated_fields: Set
    ) -> Dict[str, Any]:
        """Serializes an object to json"""
        serialized_object: Dict[str, Any] = {}
        keys_to_serialize = object_to_serialize.get_serialized_fields()
        for key in keys_to_serialize:
            
            value = getattr(object_to_serialize, key, None)
            if cls._is_excluded(value, key, object_to_serialize):
                continue
            if key in decorated_fields:
                serialized_object[key] = cls._serialize(value)
            else:
                value = cls._serialize(value)
                if isinstance(value, dict) and "__type" in value:
                    value = value["__var"]
                serialized_object[key] = value
        return serialized_object
    
    @classmethod
    def _serialize(cls, var: Any) -> Any: