
**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
"""Serializes a DAG into a JSON object."""
        try:
            serialize_dag = cls.serialize_to_json(dag, cls._decorated_fields)
            serialize_dag["tasks"] = [cls._serialize(task) for _, task in dag.task_dict.items()]
            serialize_dag['_task_group'] = SerializedTaskGroup.serialize_task_group(dag.task_group)
            
            if dag.has_on_success_callback:
                serialize_dag['has_on_success_callback'] = True
            if dag.has_on_failure_callback:
                serialize_dag['has_on_failure_callback'] = True
            return serialize_dag
        except SerializationError:
            raise
        except Exception:
            raise SerializationError(f'Failed to serialize dag {dag.dag_id!r}')
    @classmethod
    def deserialize_dag(cls, encoded_dag: Dict[str, Any]) -> 'SerializedDAG':