Maybe the present moment, of our awareness of what is now (which presumably continues to be present even when we're asleep; or nonexistent, if that's what we become), isn't a mere boundary (but is rather the whole world, a vital rather than static world)? It doesn't seem to be, but maybe that's because it doesn't seem to be an unextended instant; so, why does it seem to be extended?
The past is what has been present [and so no longer exists], the future what will be present [and so does not yet exist]: but the present is a mere durationless boundary between the past and the future, and a boundary can exist only in virtue of the existence of that which it bounds. That was Augustine’s puzzle. If you are indifferent to philosophy, you will happily ignore it; if not, you will want to know the solution to it. (Dummett, in Philosophy 2003: 392, my italics)
......Well, light could hardly be perceived within an unextended instant (since all light has nonzero wavelengths) for example, whereas the world is clearly, at this moment, illuminated. But still, that hardly means that the present must be extended, for this time has now become part of the past, as future times continually move through the present; that is, we perceive such things as light as times move continuously (so it seems) through the present, and so we've no reason to think that it must itself be extended. Now, while the past seems to us to be gone forever, maybe it still exists, in some lifeless part of existence. It might even remain known, e.g. by the Creator of this Universe (if there is one, as seems likely). And while the future is only accessible, for us, via the present collapse of the future possibilities into this actuality, maybe it and they are more directly known (similarly). So maybe it is not too odd, to think of the present instant as a boundary.
......I don't know much about time, but I do find our intuitions about it interesting. I don't know about yours (and ought to), but to me it seems that the past is like pictures or propositions; rather than, like the present, full of enduring objects. It seems to me that were the past to exist, somehow, it would be like a CD that we would have to move our attention through (like a beam of light) in order to perceive it; not so much because I think of time classically, as like a line through the space-time jelly of this world, but because when we ask "Where is this thing... now?" the answer is often "Still here," rarely "In the past."
......The past (of this world) could hardly be known, then, except as it was when it was present; but the future is clearly more unknown, in some sense (there's a sense in which it is more knowable), whence it seems unreal. And yet we do know a lot about it, e.g. that the sun will (probably) rise again tomorrow. It certainly seems that the future is less determinate than the past (and presumably the indirectly perceived present is actually the past, the directly experienced present being more of a becoming determinate), whence I almost think of it in terms of fuzzy pictures or propositions; although it also seems more real than the past. And less certain, since it isn't just the fuzziness of some enduring objects' properties, but rather that they might not (for all I know) even be there then.
......The future seems more real than the past, as it rushes to meet us, and maybe that's because it's approaching the present, with my desires directed towards it and my actions being about determining it; but furthermore, it seems that how things turn out affects our conception of what they really were, what they amounted to, were really all about (their meaning, so to speak). Anyway, in short time is to me a big mystery (whether under the assumption that we evolved naturally, or that we were created deliberately), whence I'm fascinated by what others think about it...
I urge that we must not assume that the 'existence' of time (which of course I do not deny) brings with it a well-formed philosophical question. The love of wisdom demands we be ready to question the questions that philosophy has bequeathed to us. I think that Dummett's refusal to consider questioning those questions has unfortunate consequences. It leads him to unwittingly enunciate some nonsenses. (Read, in Philosophy 2003: 402)