Ἐγώ εἰμι τὸ Ἄλφα καὶ τὸ Ὦ, λέγει Κύριος, ὁ θεός, ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος, ὁ παντοκράτωρ (WH).
"'I am the Alʹpha and the O·meʹga,' says Jehovah God, 'the One who is and who was and who is coming, the Almighty.'" (NWT 2013 Rev.)
"I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty." (KJV)
"I am the A and the Z, says the Lord God; the one who Is and Was and Is Coming; the Almighty." (Byington)
"the one who is and who was and who is coming" (Porter).
Caragounis suggests that the way Porter renders 1:8 indicates present time, but he criticizes Porter, accusing him of misunderstanding the import of Euripides, other classical Greek texts and Revelation 1:8.
Porter's error in this case is supposed to be that he thinks the aorist, present, and future tenses are not used of "past, present, and future time as some would expect. Instead, the Perfect and Present are each used for past reference, and the Present is used for future reference." See Caragounis, page 328; Porter, Verbal Aspect, 82, ftn. 5.
There is a problem with this explanation of Revelation 1:8, however, and other related texts--Caragounis would say.
He notes that ἦν is imperfect (not present) and the verb expresses past time; "ὁ ἐρχόμενος expresses future time not because it is a present participle, but because future time is natural to this verb, cf. υπαγω, etc." (328, ftn. 321).
Caragounis closes this part of his analysis, though there's more to come, with these remarks:
"The above was a critique of Porter's Introduction and first chapter, in which he tried to prove previous scholars wrong in order to establish his own theory. It was shown that Porter often fails to represent his opponents correctly, considers all scholars to have misunderstood the nature of Greek including both ancient and modern Greeks, and seems to regard himself as the only one who really knows what the verb expresses. Further, he often misunderstands and mistranslates the ancient authors" (p. 328).
I need to review Porter for myself, since I read his study in grad school, but not much since that time. Nevertheless, you get a feel for how strongly Caragounis critiques Porter's material.