"18 Children, it is the last hour! As you have heard that antichrist is coming, so now many antichrists have come. From this we know that it is the last hour."
Full context: http://www.biblegateway.com/passage/?search=1%20John+2&version=NRSV
15 Do not love the world or the things in the world. The love of the Father is not in those who love the world; 16 for all that is in the world—the desire of the flesh, the desire of the eyes, the pride in riches—comes not from the Father but from the world. 17 And the world and its desire[f] are passing away, but those who do the will of God live forever.
Warning against Antichrists
18 Children, it is the last hour! As you have heard that antichrist is coming, so now many antichrists have come. From this we know that it is the last hour. 19 They went out from us, but they did not belong to us; for if they had belonged to us, they would have remained with us. But by going out they made it plain that none of them belongs to us. 20 But you have been anointed by the Holy One, and all of you have knowledge.[g]
The majority of critical scholarship places 1 John in the 90s CE to around 100(see source), yet it clearly has a imminent sense of parousia, and proclaims that we are in the "last hour". It may be influenced by the Nero Redivivus myth. Either way, this looks like something that needs reconciling.
Raymond Brown states (An Introduction to the New Testament, pp. 389-390):
Most scholars think the Johannine Epistles were written after the Gospel. More precisely, I would place I and II John in the decade after the body of the Gospel was written by the evangelist (ca. 90) but before the redaction of the Gospel (which may have been contemporaneous with III John, just after 100). What particularly differentiates I and II John from the Gospel is the change of focus. "The Jews" who are the chief adversaries in the Gospel are absent; and all attention is on deceivers who have seceded from the community, and by so doing have shown a lack of love for their former brothers and sisters.