
**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
self,
        start_date: Optional[datetime] = None,
        end_date: Optional[datetime] = None,
        session: Session = None,
    ) -> List[TaskInstance]:
        """
        Get a set of task instance related to this task for a specific date
        range.
        """
        end_date = end_date or timezone.utcnow()
        return (
            session.query(TaskInstance)
            .filter(TaskInstance.dag_id == self.dag_id)
            .filter(TaskInstance.task_id == self.task_id)
            .filter(TaskInstance.execution_date >= start_date)
            .filter(TaskInstance.execution_date <= end_date)
            .order_by(TaskInstance.execution_date)
            .all()
        )
    def get_flat_relative_ids(