This is simply a preliminary answer, as I need to head home to help my wife prepare for Thanksgiving lunch . The citation by Peter is of Joel 2:28-32, which speaks of the coming Day of the LORD. Some have argued for a two-fold fulfillment, where the first part (Acts 2:17-18) was fulfilled in the early church and the second part (Acts 2:19-20) would be fulfilled in the final days (e.g. Revelation 6).
This seems a plausible argument to me because there are numerous Old Testament passages that juxtaposes Christ's first and second coming without any break in the prophecy, so that only in hindsight do we recognize that part of the prophecy was fulfilled in Jesus' first coming and the rest would be fulfilled when He comes again.
This confusion is demonstrated in the Gospels, where many people, including John the Baptist, thought Jesus would establish the kingdom of God right then and there. They did not understand about His two comings. Something similar may be happening in Peter's citation of Joel 2.