
**Apache Airflow version**:

master

**What happened**:

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


tests/serialization/test_dag_serialization.py
assert serialized_dag.full_filepath == dag.fileloc
    def validate_deserialized_task(
        self,
        serialized_task,
        task,
    ):
        """Verify non-airflow operators are casted to BaseOperator."""
        assert isinstance(serialized_task, SerializedBaseOperator)
        assert not isinstance(task, SerializedBaseOperator)
        assert isinstance(task, BaseOperator)
        fields_to_check = task.get_serialized_fields() - {
            
            '_task_type',
            'subdag',
            
            '_log',
            
            'template_fields',
            
            'on_failure_callback',
            'on_success_callback',
            'on_retry_callback',
            
            'resources',
        }
        assert serialized_task.task_type == task.task_type
        assert set(serialized_task.template_fields) == set(task.template_fields)
        assert serialized_task.upstream_task_ids == task.upstream_task_ids
        assert serialized_task.downstream_task_ids == task.downstream_task_ids
        for field in fields_to_check: