
**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
"""Validate pickle in a subprocess."""
    dags = collect_dags(dag_folder)
    for dag in dags.values():
        queue.put(SerializedDAG.to_json(dag))
    queue.put(None)
class TestStringifiedDAGs(unittest.TestCase):
    """Unit tests for stringified DAGs."""
    def setUp(self):
        super().setUp()
        BaseHook.get_connection = mock.Mock(
            return_value=Connection(
                extra=(
                    '{'
                    '"project_id": "mock", '
                    '"location": "mock", '
                    '"instance": "mock", '
                    '"database_type": "postgres", '
                    '"use_proxy": "False", '
                    '"use_ssl": "False"'
                    '}'
                )
            )
        )
        self.maxDiff = None  
    def test_serialization(self):