Heidegger's Bible Handbook: 1 Samuel: Chronology
4. An account of the time. The affairs of eighty years are narrated.
The affairs that are narrated here are of eighty years. For Eli judged the people for forty years, as it is evident out of 1 Samuel 4:18. But, that from Samuel to the death of Saul another forty years passed by, is indicated in Acts 13:21.