
**Apache Airflow version**:

master

**What happened**:

https://github.com/apache/airflow/blob/master/airflow/models/baseoperator.py#L356 --> should be timedelta not datetime


airflow/models/baseoperator.py
"""Stringified DAGs and operators contain exactly these fields."""
        if not cls.__serialized_fields:
            from airflow.models.dag import DagContext
            
            
            
            
            DagContext.push_context_managed_dag(None)
            cls.__serialized_fields = frozenset(
                vars(BaseOperator(task_id='test')).keys()
                - {
                    'inlets',
                    'outlets',
                    '_upstream_task_ids',
                    'default_args',
                    'dag',
                    '_dag',
                    '_BaseOperator__instantiated',
                }
                | {
                    '_task_type',
                    'subdag',
                    'ui_color',
                    'ui_fgcolor',
                    'template_fields',
                    'template_fields_renderers',
                }