The Bible is a prophetic book. That alone is an amazing statement, because it is the only prophetic book in the world, since it is the only one written by God. Prophecy has a lot of purposes, a major one being a validation that that the Bible is in fact the Word of God. As you open the New Testament, it is easy to see the importance of prophecy all over it. God wants us to take it seriously.
The first page of the New Testament in Matthew, a genealogy, is related to prophecy, because the Abrahamic and Davidic covenants are prophetic. The genealogy proves that Jesus is a fulfillment of those predictions. Then you get the Isaiah 7:14 prophecy that says that Jesus is a fulfillment of that. Then you have the magi setting off looking for the Messiah based upon what? Prophecy. Then there are four wondrous prophecies in four different geographical location in the second half of Matthew 2 that confirm who Jesus is. Matthew 3 talks about John the Baptist, himself another fulfillment of prophecy.
When Peter preaches on the Day of Pentecost, almost every point he makes relies on prophecy. When the baptism of the Holy Spirit occurs, what is that? It is a fulfillment of the prophecy of John the Baptist, Acts 1:5, which is repeated by Jesus before He ascends into heaven. When the unbelievers mock what's happening in Acts, Peter defends it with what? Prophecy. He refers to Joel 2:28-32 in Acts 2:17-21 to kick off his sermon there, explaining to the audience what's going on. He starts:
15 For these are not drunken, as ye suppose, seeing it is but the third hour of the day. 16 But this is that which was spoken by the prophet Joel; 17 And it shall come to pass in the last days, saith God
It is such an unusual, outlying event, outside of the norm for comprehension, Peter makes the connection to the Old Testament. This gigantic crowd wasn't all drunken. This is what Joel was talking about, and Peter says that what was occurring there on the Day of Pentecost was "in the last days." Generally, when people say, "We're in the last days," they mean something different than what Peter says, so that becomes confusing. Peter's usage of the last days is the correct usage and it's what we should imitate.
We're not waiting for the last days. We're already in them. Peter was saying that he and his audience were in them. 1 John 2:18 says,
Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time.
"Last days" or "last time," which is the same terminology, is ironically a terminology from Old Testament prophecy. That's what is supposed to get us up to speed is the Old Testament usage. Here are some places:
Isaiah 2:2, And it shall come to pass in the last days, that the mountain of the LORD'S house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it.
Jeremiah 23:20, The anger of the LORD shall not return, until he have executed, and till he have performed the thoughts of his heart: in the latter days ye shall consider it perfectly.
Ezekiel 38:8, After many days thou shalt be visited: in the latter years thou shalt come into the land that is brought back from the sword, and is gathered out of many people, against the mountains of Israel, which have been always waste: but it is brought forth out of the nations, and they shall dwell safely all of them.Daniel 10:14, Now I am come to make thee understand what shall befall thy people in the latter days: for yet the vision is for many days.
And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.