
**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
"""Serialisation / deserialisation continues to work without kubernetes installed"""
    def mock__import__(name, globals_=None, locals_=None, fromlist=(), level=0):
        if level == 0 and name.partition('.')[0] == 'kubernetes':
            raise ImportError("No module named 'kubernetes'")
        return importlib.__import__(name, globals=globals_, locals=locals_, fromlist=fromlist, level=level)
    with mock.patch('builtins.__import__', side_effect=mock__import__) as import_mock:
        
        
        spec = importlib.util.find_spec("airflow.serialization.serialized_objects")
        module = importlib.util.module_from_spec(spec)
        spec.loader.exec_module(module)
        
        
        imported_airflow = {
            c.args[0].split('.', 2)[1] for c in import_mock.call_args_list if c.args[0].startswith("airflow.")
        }
        assert "kubernetes" not in imported_airflow
        
        pod_override = {
            '__type': 'k8s.V1Pod',
            '__var': PodGenerator.serialize_pod(executor_config_pod),
        }
        with pytest.raises(RuntimeError):
            module.BaseSerialization.from_dict(pod_override)
        
        module.SerializedDAG.to_dict(make_simple_dag()["simple_dag"])