The English future perfect in English could be considered "the past in the future." That is, it refers to something that is in the past from the viewpoint of the future. It expresses an action that will have finished at a certain time in the future.
- She is sure that the package will have arrived by his birthday.
(Talking now, looking back from his birthday to the arrival of the package.)
- It is our golden wedding next year, so we will have been married for 50 years.
(Talking now, looking back from our golden wedding to when we got married.)