It is sometimes said that Augustine held a view of time in which only the present exists, on the basis of what he says in the Confessions. I would like to suggest that Augustine is actually engaged in discussing a much more fundamental issue, one that must arise for proponents of the 'A-series' or the 'B-series' alike. Considering this issue, which has to do with temporal measurement, raises interesting questions for both views of time, and suggests a possible third position.
The discussion of time in Book XI of the Confessions first sets out the problem in a general way:
Yet I say boldly that I know that if nothing passed away there would be no time past. And if nothing were coming, there would be no future time. And if there were nothing, there would be no present time. Those two times, then, past and future--how are they, when the past is no longer and the future is not yet? But should the present always be present and never pass into time past, truly it would not be time, but eternity. (241-242)
As Augustine further develops this line of thought, it is clear that the issue that concerns him is bound up in the fact that we measure intervals of time. We call soem times short and some times long, whether those times are said to be past, present, or future. The question then arises, "in what sense is something long or short which does not exist? For the past is not now, and the future is not yet" (242). Augustine considers a response to this which says taht we should not say the past or the future "is" long, but "has been" or "will be" long; but, as he notes, this does not actually remove the real point at issue. The same basic question arises: In what sense can we ascribe extension ("long" or "short") to any time? Someone might be tempted to say that it had or will have extension when it was or will be present, but once again this fails to evade the major issue involved. How are we to take this present?
If any portion of time is conceived, which cannot now be divided into even the minutest particles of moments, that alone is what may be called present. And that flies by with such speed from future to past tthat it cannot be lengthened out in the least, for if it is extended, it is divided between past and future. The present has no extension or length. (243)
The trouble with all these attempts to geta round the problem is that all our measuring of time is itself time-conditioned. Unlike spatial measurement, in time measurement we cannot stand outside what we are measuring and stretch some measuring device along the whole of its extension. Our measurements of time cannot occur except entirely at a particular time. This is a problem any view of tiem will have to face. Suppose we mentally mark out when a race begins and when it ends. We cannot, as we could with spatial measurement, step back and compare the two marks, because we do nto have access to the temporal marks that we do with spatial marks. Any comparison of the beginning and ending mark will not be a direct comparison of marks. All our measurements of teh extension of time occur entirely in some present or at some time t. We seem to have no way of measuring time (in any clear sense of the word 'measuring') at all. It is nevertheless certainly true that we perceive and compare intervals of time. The question Augustine raises is that of how we do so:
We even measure how much longer or shorter this time is than that; and we answer,"This is double, or treble, while this other is but once, or only just as long as that." But we measure times as they are passing, by perceiving them. But past times, which no longer are, or future times, which are not yet, who can measure? Unless, perhaps, anyone would dare to say that what is not can be measured. When, therefore, time is passing, it can be perceived and measured; but when it is past, it cannot, because it is not. (244)
This may sound something like the claim that only the present exists (where 'exists' is presumably understood tenselessly). I doubt, however, that htis can be any more than an anachronistic reading. Augustine, of course, is nto familiar with McTaggart's A-series/B-series distinction; and the most natural way of reading Augsutine's claim is simply that times are only when they are; and when they are what we call 'past' or 'future' they either no longer are, or are not yet--truisms on any perspective. Indeed, any theory of time that somehow failed to encapsulate these points woul be nothing more than the denial that there really is time--i.e., that there really is a difference between any given 'now' and 'then'. The 'now' or the 'then', of course, may be fixed in either A-series style or B-series style. In any case, denying that only the present exists would not remove the Augustinian problem; how we measure an interval of time without some sort of direct access to the whole interval remains a question. Even if Augustine were assuming an extreme view of time, the problem (and, I think, the solution) he gives are not limited to that view, because it is a much more fundamental problem than the problems that generate these different views. There is, however, evidence in the text that Augustine is not so clearly committed to the claim that only the present exists (taken tenselessly), since in several places he seriously considers the idea that past and future really are, and after much discussion denies that they do in a way that shows that he would take the 'exists' in "only the present exists" to be simply present-tensed, not tenseless. In particular, he notes that our memory's access to the past does nto give us access to past things, but only to present traces of things past, "words which are conceived from the images of things which they have left as traces in the mind in their passage through the senses" (245). Our measurement of the past and the future as if they were present is not an access to the past and the future as themselves present; hence the problem.