
**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
"""OperatorLink not registered via Plugins nor a built-in OperatorLink"""
            name = 'My Link'
            def get_link(self, operator, dttm):
                return 'https://www.google.com'
        class MyOperator(BaseOperator):
            """Just a DummyOperator using above defined Extra Operator Link"""
            operator_extra_links = [TaskStateLink()]
            def execute(self, context):
                pass
        with DAG(dag_id='simple_dag', start_date=datetime(2019, 8, 1)) as dag:
            MyOperator(task_id='blah')
        serialized_dag = SerializedDAG.to_dict(dag)
        with self.assertLogs("airflow.serialization.serialized_objects", level="ERROR") as log_output:
            SerializedDAG.from_dict(serialized_dag)
            received_logs = log_output.output[0]
            expected_err_msg = (
                "Operator Link class 'tests.serialization.test_dag_serialization.TaskStateLink' "
                "not registered"
            )
            assert expected_err_msg in received_logs
    def test_extra_serialized_field_and_multiple_operator_links(self):