
**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
"""A JSON serializable representation of operator.
    All operators are casted to SerializedBaseOperator after deserialization.
    Class specific attributes used by UI are move to object attributes.
    """
    _decorated_fields = {'executor_config'}
    _CONSTRUCTOR_PARAMS = {
        k: v.default
        for k, v in signature(BaseOperator.__init__).parameters.items()
        if v.default is not v.empty
    }
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        
        
        self._task_type = 'BaseOperator'
        
        self.ui_color = BaseOperator.ui_color
        self.ui_fgcolor = BaseOperator.ui_fgcolor
        self.template_fields = BaseOperator.template_fields
        self.operator_extra_links = BaseOperator.operator_extra_links
    @property
    def task_type(self) -> str:
        
        
        return self._task_type
    @task_type.setter
    def task_type(self, task_type: str):
        self._task_type = task_type
    @classmethod
    def serialize_operator(cls, op: BaseOperator) -> Dict[str, Any]: