
**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
return dag.following_schedule(execution_date)
    default_args = {'start_date': datetime(2019, 7, 10)}
    dag = DAG(
        'user_defined_macro_filter_dag',
        default_args=default_args,
        user_defined_macros={
            'next_execution_date': compute_next_execution_date,
        },
        user_defined_filters={'hello': lambda name: f'Hello {name}'},
        catchup=False,
    )
    BashOperator(
        task_id='echo',
        bash_command='echo "{{ next_execution_date(dag, execution_date) }}"',
        dag=dag,
    )
    return {dag.dag_id: dag}
def collect_dags(dag_folder=None):