
**Apache Airflow version**:

master

**What happened**:

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


airflow/serialization/serialized_objects.py
"""Deserializes a DAG from a JSON object."""
        dag = SerializedDAG(dag_id=encoded_dag['_dag_id'])
        for k, v in encoded_dag.items():
            if k == "_downstream_task_ids":
                v = set(v)
            elif k == "tasks":
                
                SerializedBaseOperator._load_operator_extra_links = cls._load_operator_extra_links
                
                v = {task["task_id"]: SerializedBaseOperator.deserialize_operator(task) for task in v}
                k = "task_dict"
            elif k == "timezone":
                v = cls._deserialize_timezone(v)
            elif k in {"dagrun_timeout"}:
                v = cls._deserialize_timedelta(v)
            elif k.endswith("_date"):
                v = cls._deserialize_datetime(v)
            elif k in cls._decorated_fields:
                v = cls._deserialize(v)
            
            setattr(dag, k, v)
        
        
        if "_task_group" in encoded_dag:
            dag._task_group = SerializedTaskGroup.deserialize_task_group(  
                encoded_dag["_task_group"], None, dag.task_dict
            )
        else: