
**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
from unittest import mock
import pytest
from dateutil.relativedelta import FR, relativedelta
from kubernetes.client import models as k8s
from parameterized import parameterized
from airflow.hooks.base import BaseHook
from airflow.kubernetes.pod_generator import PodGenerator
from airflow.models import DAG, Connection, DagBag, TaskInstance
from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
from airflow.operators.bash import BashOperator
from airflow.security import permissions
from airflow.serialization.json_schema import load_dag_schema_dict
from airflow.serialization.serialized_objects import SerializedBaseOperator, SerializedDAG
from tests.test_utils.mock_operators import CustomOperator, CustomOpLink, GoogleLink
executor_config_pod = k8s.V1Pod(
    metadata=k8s.V1ObjectMeta(name="my-name"),
    spec=k8s.V1PodSpec(
        containers=[
            k8s.V1Container(name="base", volume_mounts=[k8s.V1VolumeMount(name="my-vol", mount_path="/vol/")])
        ]
    ),
)
serialized_simple_dag_ground_truth = {
    "__version": 1,
    "dag": {
        "default_args": {
            "__type": "dict",
            "__var": {
                "depends_on_past": False,
                "retries": 1,