Let's say there are two people with Death Notes, John Zeürtö and Bob Schnittlauchkäsebrot.
John Zeürtö writes:
Bob Schnittlauchkäsebrot dies 23 days after the latter willingly forfeits ownership of the death note.
What would happen? Some guesses:
Bob dies only after 23 days after he gives up the ownership of the death note, if he does or until his lifespan runs out.
Bob gets killed in 23 days.
Bob gives up ownership of his death note and then gets killed in 23 days.
Bob dies of a heart attack.