In an use case model the actor is someone or something outside the system that interacts with the system. An actor may be a person, a device, another system or sub-system. The actor is represented as a “stickman” icon.
So what should we do when we want a use case is automatically started on a certain moment? Probably started by some scheduling mechanism.
According to the definition of an actor we need to know if the scheduling mechanism is a part of the system or something outside the system. Is it part of the system then you don’t model a actor. Is it something outside the system you do model the actor (with the name of that scheduling system)
But when modeling use cases we don’t know or we don’t want to know about how the system is implemented. So it’s not clear if we need to model the actor.
I prefer that all user goal level use cases are always started by an actor. Then I can easily check of my model is complete: every use case needs to be associated with an actor or with another use cases. If not the use cases is useless and can be removed from your model.
My recommendation is always model a Time actor for scheduled use cases also when the scheduling is implemented inside the system. It makes your model clear and complete.