Does it really matter which books were written when?
Isn’t it more important that John wrote both of them?
There's no clear consensus as to which John wrote the Apocalypse, or even as to how many Johns there are.
We can speak of three Johns, though it may only be two Johns, or even all the one and the same John:
1. John the Apostle
2. John the Presbyter
3. John the Revelator
It is somewhat interesting that of all the texts which bear the name of "John", only one of those texts identifies its author as "John". The Gospel and three Epistles that bear the name of John are nameless. The Evangelist only identifies himself as "the disciple whom Jesus loved", 1 John contains no self-designation for the author, and 2 and 3 John identify the author only as "the presbyter".
In time the tradition(s) concerning the authorship of these works, and the identify of the John behind these works seems to have coalesced into the identity of St. John the Apostle, and as such the Apostle is given credit as the author of all five of the Johannine works.
This isn't too uncommon, the same sorts of things appear to have happened with other figures. For example the Gospels mention a lot of different women named Mary. So in the Western tradition, as an example, Mary of Bethany (the sister of Lazarus and Martha) is the same as Mary Magdalene, and further in the West Mary Magdalene came to be associated with the penitent prostitute. Though in the East these are three completely different women. Mary Magdalene and Mary of Bethany are completely different Marys, and the unnamed prostitute is someone completely different as well.
It's not always easy with these things, and when we start looking at various strands of tradition--especially competing, differing traditions--trying to put them all together in a nice and tidy way usually just doesn't work; and so we are left with a lot that is simply unknown.
-CryptoLutheran