
**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
"""Collects DAGs to test."""
    dags = {}
    dags.update(make_simple_dag())
    dags.update(make_user_defined_macro_filter_dag())
    if dag_folder:
        if isinstance(dag_folder, (list, tuple)):
            patterns = dag_folder
        else:
            patterns = [dag_folder]
    else:
        patterns = [
            "airflow/example_dags",
            "airflow/providers/*/example_dags",
            "airflow/providers/*/*/example_dags",
        ]
    for pattern in patterns:
        for directory in glob(f"{ROOT_FOLDER}/{pattern}"):
            dags.update(make_example_dags(directory))
    
    dags = {dag_id: dag for dag_id, dag in dags.items() if not dag.is_subdag}
    return dags
def serialize_subprocess(queue, dag_folder):