ooad1-3  Exercise 2: A Diagram With a Timer Actor
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overdue.usecase.puml (PlantUML 1.2026.8):

  @startuml
  scale 1.4
  left to right direction

  actor Member
  actor Librarian
  actor "Overnight\nTimer" as Timer <<time>>

  rectangle "Library lending system" {
    usecase "Borrow a book" as Borrow
    usecase "Return a book" as Return
    usecase "Send overdue notice" as Notice
    usecase "Pay a fine" as PayFine
  }

  Member --> Borrow
  Member --> Return
  Member --> PayFine
  Librarian --> Borrow
  Librarian --> Return
  Timer --> Notice
  Notice --> Member
  @enduml

Render and output:

  rendered    ex2\overdue.usecase.puml  ->  overdue.usecase.puml.svg  (7 KB)

What you should see: Librarian and Member on the left of the boundary and the Overnight Timer (tagged <<time>>) below them; four use cases inside the rectangle; the timer connects to "Send overdue notice", which connects to the Member.

Primary or supporting? The Timer is the PRIMARY actor of "Send overdue notice": it triggers the use case, and the goal (notices go out) is the library's. The Member is not a primary actor, because the Member did not start it and has no goal in it; the Member is a recipient, which some authors call a secondary or receiving actor. The Member's own goal appears elsewhere ("Pay a fine"), so the notice use case has a stakeholder (the Member, who wants a clear message) that is not driving it.

WHY THIS WORKS AS AN ANSWER
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A time-triggered use case has no human at the start, which is easy to forget when you only ask "what do users want?". Making the timer an actor keeps the rule that every use case has a primary actor and that actors are outside the scope; the scheduler is not part of the lending system's logic, it merely calls it.