The following is found also in the "author/date" section of 1 & 3 John.
There is nothing within 2 or 3 John that points unambiguously towards a specific author. Irenaeus of Lyons in the early 2nd Century links 2 John with 1 John, and ascribes them both to John the Apostle and author of the fourth gospel. It appears that Origen was the first to mention all three, though according to Eusebius, he mentioned the second and third whilst pointing out that not all viewed them as authentic (Eusebius, op. cit., vi, 25).
Arguments that 2 and 3 John came from a different author than that of 1 John become evidently problematic when considering content and vocabulary, as several distinct thematic links exist between the first epistle and the last two. Much of the foundation of the view of separate authorship comes from the self-identification of the writer of 2 and 3 John within the text as ‘the elder.’
Many scholars (including Eusebius) view it as unlikely that John the Apostle would refer to himself as ‘the elder’, yet it is not impossible. In many ways, it would make sense, as the Apostle John was the last apostle left alive and reputedly lived to an old age. It would then be quite natural to refer to himself as ‘the elder’ as he truly was in a special position. If, on the other hand he had referred to himself as ‘the Apostle’, simply because he was the last apostle alive, this would have been "both presumptuous and out of character".
1 John is unanimously agreed by the early Church to have been written by John son of Zebedee (the Apostle), and it seems that the obvious similarities in content of 2 & 3 John with 1 John point towards his authorship of these as well.
Probably written during the early nineties - though it is impossible to be sure.