I dedicate this post to that loveable rascal Bill Clinton who taught us just how much can ride on what the meaning of 'is' is.
Credit where credit is due: Some of the inspiration for this post comes from a conversation with Peter Lupu and from an article he recommended, S. Savitt, Presentism and Eternalism in Perspective.
1. There is first of all the 'is' of atemporality. Assuming that there are timeless entities such as God (concrete) and the number 13 (abstract), any sentences we use to talk about them must feature tenseless verbs and copulae. Consider the proposition expressed by the sentence, '13 is prime.' 13 is prime, but not now and not always. If the truth were always true, it would be in time. The truth is timeless and so is the object 13 and the property of being prime. The same goes for '13 exists.' It is not true now nor at every time. It is true timelessly. It is worth noting that the timeless is' and 'exists' do not abstract from the temporal determinations of pastness, presentness, and futurity for the simple reason that numbers and such are not in time in the first place. So the 'is' of atemporality is not the result of a de-tensing operation whereby we abtract from the temporal determinations to lay bare the pure copula, the copula that merely 'copulates.' The 'is' in question is tenseless from the 'git-go.'
Perhaps we should distinguish between grammatical tense and logical tense. Every verb has a grammatical tense. Thus the verb in 'God exists' is in the present tense. But God exists timelessly, and so 'exists' in this instance is logically without a tense.
Consider John 8:58: "Before Abraham was, I am." Is that ungrammatical? Yes, but logically it makes sense.
2. At the opposite end of the spectrum we find the 'is' of temporal presentness. Examples: 'Peter is smoking' and 'There are 13 donuts in the box.' There are now 13 donuts in the box.
3. The 'is' of omnitemporality. Savitt gives the example of 'Copper is a conductor of electricity.' The sentence is true at every time, not just at present. But it is not timelessly true since it is about something in time, copper. I think the example shows that the tenseless is not the same as the timeless. What is timelessly true is tenselessly true, but not conversely.
4. The Disjunctively Detensed 'Is.' We can de-tense 'is' as follows: x is detensedly F just in case x was F or is F or will be F. We can do the same with 'exists.' Thus, Socrates is detensedly wise iff Socrates was wise or is wise or will be wise. De-tensing involves abstracting from temporal determinations. A detensed copula is a pure copula: all it does is 'copulate' or link.
The 'am' in 'I am dead' is a pure copula, and the sentence is tenselessly true, but not presently true or timelessly true or omnitemporally true. Gott sei dank!
5. The Hypertenseless 'Is.' God exists atemporally and thus tenselessly while Socrates exists temporally but not presently or omnitemporally and thus he too exists tenselessly. If there is a hypertenseless sense of 'exist' it applies to both God and Socrates and abstracts from the way each exists, atemporally in the case of God, temporally in the case of Socrates.
In 'God and Socrates both exist,' the 'exist' is hypertenseless in that it is abstractly common to both the tenselessness of the 'exists' in 'God exists' and the tenselessness of the 'exists' in 'Socrates exists.'
Now what is this hypertenseless univocal sense of 'exists' that applies to both God and Socrates? Persumably it is the quantifier sense according to which x exists iff (Ey) x = y. Existence in this sense is identity-with-something-or-other. Absolutely everything, whatever its mode of existence, exists in this hypertenseless sense.
Now the presentist wants to say that, necessarily, it is always the case that only present items exist. But in what sense of 'exist'? It cannot be the first four, for reasons given in previous posts. So let's try the fifth sense. Accordingly, only present items are identical-with-something-or-other.
Does this work?