
**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
"retry_delay": {"__type": "timedelta", "__var": 300.0},
                "sla": {"__type": "timedelta", "__var": 100.0},
            },
        },
        "start_date": 1564617600.0,
        '_task_group': {
            '_group_id': None,
            'prefix_group_id': True,
            'children': {'bash_task': ('operator', 'bash_task'), 'custom_task': ('operator', 'custom_task')},
            'tooltip': '',
            'ui_color': 'CornflowerBlue',
            'ui_fgcolor': '#000',
            'upstream_group_ids': [],
            'downstream_group_ids': [],
            'upstream_task_ids': [],
            'downstream_task_ids': [],
        },
        "is_paused_upon_creation": False,
        "_dag_id": "simple_dag",
        "fileloc": None,
        "tasks": [
            {
                "task_id": "bash_task",
                "owner": "airflow",
                "retries": 1,
                "retry_delay": 300.0,
                "sla": 100.0,
                "_downstream_task_ids": [],
                "_inlets": [],
                "_is_dummy": False,