
**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
try:
                children = node.children.values()
            except AttributeError:
                
                expected_serialized = SerializedBaseOperator.serialize_operator(dag.get_task(node.task_id))
                expected_deserialized = SerializedBaseOperator.deserialize_operator(expected_serialized)
                expected_dict = SerializedBaseOperator.serialize_operator(expected_deserialized)
                assert node
                assert SerializedBaseOperator.serialize_operator(node) == expected_dict
                return
            for child in children:
                check_task_group(child)
        check_task_group(serialized_dag.task_group)
    @parameterized.expand(
        [
            ("poke", False),
            ("reschedule", True),
        ]
    )
    def test_serialize_sensor(self, mode, expect_custom_deps):
        from airflow.sensors.base import BaseSensorOperator
        class DummySensor(BaseSensorOperator):