"The mayor will close his office."
Translation:Borgmesteren vil lukke sit kontor.
Without a specified time, one has to assume that "Borgmesteren lukker sit kontor" is happening right now. If he were to do so tomorrow, you could say: "Borgmesteren lukker sit kontor i morgen" and everyone would know that we were talking about the future.
In this case, he will close his office in the future, so we need "vil lukke" (= future) in Danish.