In the last book of the Old Testament (Malachi), we are told the following

(Mal 4:5) "Behold, I am going to send you Elijah the prophet before the coming of the great and terrible day of the LORD.

Who is the "you" in this verse. The Jews! Malachi was a prophet to the Jews. Has this Elijah come? Yes

(Mat 11:12) "And from the days of John the Baptist until now the kingdom of heaven suffers violence, and violent men take it by force. (Mat 11:13) "For all the prophets and the Law prophesied until John[the baptist]. (Mat 11:14) "And if you care to accept it, he himself is Elijah, who was to come.

(Mat 17:12) but I say to you, that Elijah already came, and they did not recognize him, but did to him whatever they wished. So also the Son of Man is going to suffer at their hands." (Mat 17:13) Then the disciples understood that He had spoken to them about John the Baptist.

John came in the "spirit" of Elijah, and according to Jesus himself he is the fulfillment of this prophecy. This is important for two reasons. First, it proves that prophecy can be fulfilled literally, although done spiritually and different than we might think or understand. Second, John came preaching repentance and judgment to the Jews prior to the great and terrible day of the Lord, AD70. This further establishes that the last days and/or the day of the Lord were in that first century time frame, not ours. Malachi was a prophet to the Jews telling of future destruction for their apostasy from God. Four-hundred years later John the Baptist comes on the scene and fulfills this prophecy in the spirit of Elijah and preaches the nearness of this wrath and judgment. This is a prophecy to the Jews, not for any future generation.