
**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
"""Helper function of depth first search for serialization.
        The serialization protocol is:
        (1) keeping JSON supported types: primitives, dict, list;
        (2) encoding other types as ``{TYPE: 'foo', VAR: 'bar'}``, the deserialization
            step decode VAR according to TYPE;
        (3) Operator has a special field CLASS to record the original class
            name for displaying in UI.
        """
        if cls._is_primitive(var):
            
            if isinstance(var, enum.Enum):
                return var.value
            return var
        elif isinstance(var, dict):
            return cls._encode({str(k): cls._serialize(v) for k, v in var.items()}, type_=DAT.DICT)
        elif isinstance(var, list):
            return [cls._serialize(v) for v in var]
        elif HAS_KUBERNETES and isinstance(var, k8s.V1Pod):
            json_pod = PodGenerator.serialize_pod(var)
            return cls._encode(json_pod, type_=DAT.POD)
        elif isinstance(var, DAG):
            return SerializedDAG.serialize_dag(var)
        elif isinstance(var, BaseOperator):
            return SerializedBaseOperator.serialize_operator(var)