
**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
"""Serializes operator into a JSON object."""
        serialize_op = cls.serialize_to_json(op, cls._decorated_fields)
        serialize_op['_task_type'] = op.__class__.__name__
        serialize_op['_task_module'] = op.__class__.__module__
        
        serialize_op['_is_dummy'] = op.inherits_from_dummy_operator
        if op.operator_extra_links:
            serialize_op['_operator_extra_links'] = cls._serialize_operator_extra_links(
                op.operator_extra_links
            )
        if op.deps is not BaseOperator.deps:
            
            
            
            deps = []
            for dep in op.deps:
                klass = type(dep)
                module_name = klass.__module__
                if not module_name.startswith("airflow.ti_deps.deps."):
                    raise SerializationError(
                        f"Cannot serialize {(op.dag.dag_id + '.' + op.task_id)!r} with `deps` from non-core "
                        f"module {module_name!r}"
                    )
                deps.append(f'{module_name}.{klass.__name__}')
            serialize_op['deps'] = deps
        
        
        if op.template_fields:
            for template_field in op.template_fields: