
**Apache Airflow version**:

master

**What happened**:

https://github.com/apache/airflow/blob/master/airflow/models/baseoperator.py#L356 --> should be timedelta not datetime


airflow/models/baseoperator.py
'on_execute_callback',
        'on_failure_callback',
        'on_success_callback',
        'on_retry_callback',
        'do_xcom_push',
    }
    
    supports_lineage = False
    
    __instantiated = False
    
    _lock_for_execution = False
    
    @apply_defaults
    def __init__(
        self,
        task_id: str,
        owner: str = conf.get('operators', 'DEFAULT_OWNER'),
        email: Optional[Union[str, Iterable[str]]] = None,
        email_on_retry: bool = conf.getboolean('email', 'default_email_on_retry', fallback=True),
        email_on_failure: bool = conf.getboolean('email', 'default_email_on_failure', fallback=True),
        retries: Optional[int] = conf.getint('core', 'default_task_retries', fallback=0),
        retry_delay: timedelta = timedelta(seconds=300),
        retry_exponential_backoff: bool = False,
        max_retry_delay: Optional[datetime] = None,
        start_date: Optional[datetime] = None,
        end_date: Optional[datetime] = None,
        depends_on_past: bool = False,
        wait_for_downstream: bool = False,
        dag=None,