It challenges the use of the external evidence (such as Irenaeus and Clement of Alexandria) often used for dating Mark, relying instead on internal evidence from the gospel itself.
D., and even most conservative scholars dating at least Revelation around 95 A. Of course John the son of Zebedee, the disciple of Jesus, could not have lived long enough to write anything much into the second century, so in this case establishing a date of writing should first involve establishing that John was in fact the author.
There is little dispute as to a common author for the short letters of 2 John and 3 John.
Both are written by a man calling himself The Elder.
The Greek of Revelation is different and non-standard, probably because John wrote it as a letter without help.
The Gospel of John, though clearly coming from John, looks like it was a collaberative effort.