
**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
"_outlets": [],
                "ui_color": "#f0ede4",
                "ui_fgcolor": "#000",
                "template_fields": ['bash_command', 'env'],
                "template_fields_renderers": {'bash_command': 'bash', 'env': 'json'},
                "bash_command": "echo {{ task.task_id }}",
                'label': 'bash_task',
                "_task_type": "BashOperator",
                "_task_module": "airflow.operators.bash",
                "pool": "default_pool",
                "executor_config": {
                    '__type': 'dict',
                    '__var': {
                        "pod_override": {
                            '__type': 'k8s.V1Pod',
                            '__var': PodGenerator.serialize_pod(executor_config_pod),
                        }
                    },
                },
            },
            {
                "task_id": "custom_task",
                "retries": 1,
                "retry_delay": 300.0,
                "sla": 100.0,
                "_downstream_task_ids": [],
                "_inlets": [],
                "_is_dummy": False,
                "_outlets": [],
                "_operator_extra_links": [{"tests.test_utils.mock_operators.CustomOpLink": {}}],