
**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
"ui_color": "#fff",
                "ui_fgcolor": "#000",
                "template_fields": ['bash_command'],
                "template_fields_renderers": {},
                "_task_type": "CustomOperator",
                "_task_module": "tests.test_utils.mock_operators",
                "pool": "default_pool",
                'label': 'custom_task',
            },
        ],
        "timezone": "UTC",
        "_access_control": {
            "__type": "dict",
            "__var": {
                "test_role": {
                    "__type": "set",
                    "__var": [permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT],
                }
            },
        },
    },
}
ROOT_FOLDER = os.path.realpath(
    os.path.join(os.path.dirname(os.path.realpath(__file__)), os.pardir, os.pardir)
)
def make_example_dags(module_path):
    """Loads DAGs from a module for test."""
    dagbag = DagBag(module_path)
    return dagbag.dags
def make_simple_dag():