Heidegger's Bible Handbook: John: Time of Writing
7. The time of writing.
With respect to the time of writing, that he wrote after the other Evangelists, even indeed at an old and infirm age, the ancients have unanimously related. Theophylact has it that he wrote on the island of Patmos about thirty years after the ascension. Likewise the Arabic Codex of Kirstenius, page 51. But, that he wrote after his return from the island of Patmos, while he was living at Ephesus, in his ninety-seventh year, Irenæus in Against Heresies, book III, section 1, Athanasius in his Synopsi, Hippolytus of Portus in Cedrenus on Nerva, and Ephiphanius on hæresi 51, relate; whose tradition is certainly the more likely.
 George Cedrenus was an eleventh century Byzantine historian. His Concise History of the World begins at the creation, and runs all the way through to his own day.  Nerva reigned from 96 to 98 AD.