Write the things which "THOU HAST SEEN"(past tense to John when he received Rev.),,,and things "WHICH ARE"(present tense to John when he received Rev.),,,and the things "WHICH SHALL BE HEREAFTER",,,(Revelations 1:19 KJV)
Some say the Revelation is dated about ad96, some date before the destruction of the temple in July ad70. but the fact(to us) is the same that is,,,
If this Revelation was given to John around ad70-96 (AND) Revelations 1:19 KJV states that what John was writing is/are things he had seen,things that are,and things that will come to pass then it is broken down into THREE GROUPS,,,past, present and future (in aspect to John when he was on Patmos around 2000 years ago)...
So there are a set of groups of things written in Revelations,,,
(1) things that had already happened,that John had already seen take place at the time of the writing(approx.ad95). i.e. things that were already historical and were being written from a past tense stance.
(2) things that are(in aspect to the timing when John was given this),i.e. the current things happening in the world during the days when John was on Patmos (present tense to John in those days)...
(3) things which shall be hereafter,,,future tense prophecy in aspect to the time when John was on Patmos i.e. the things that had not yet taken place in "that day" and were to be fulfilled in the future(between ad96 and now/approx.)
So if there are three set's of tenses that are told to John to be written what parts of Revelations were happening in John's days(that are recorded in Revelations),,,which things had already taken place when John was told to write this. And then the remainder are the future tense portion of the Revelation.
p.s. the "apostolic fathers" writings in my opinion also bare on this matter because they record a historical witness to the events that shortly take place and many of their writers give opinions as to weather they thought these certain things were to still take place,or if they were already fulfilled in aspect to the historical witness of the things/events taking place during their lives.