Consider three debates: solipsism vs. other minds; presentism vs. eternalism; actualism vs. extreme modal realism. Let say that, like most people, we want to go for other minds and actualism. The sane view is that of course other people exist but unicorns don't. Can we get any guidance from this decision as to the presentism vs. eternalism debate? Is "now" more like "I", in which case we get the hint that we should be eternalists, or is "now" more like "actual", in which case the hint is that we should be presentists?
Here is one important way in which "now" is more like "I". I communicate with people who are other than I. I do not communicate with people who are other than actual. But I do communicate with people who are other than now: I read Plato and maybe even aspire to writing for people yet to be conceived. And even when we communicate with people who are now alive, typically—unless we're speaking at each other at the same time—we do so diachronically. I speak now and they will respond later. I respond now and they spoke earlier. So our conversation reaches across times, just as it reaches across people. But it does not reach across worlds.
Suppose we think of sentences like "I am sitting" as expressing self-locating propositions (I actually probably don't want to think of them like that). Here, then, is a closely related point, inspired (as really is the above stuff) by this article. I tell you on the phone: "I am sitting." In so doing, I express a de se self-locating proposition, a proposition that locates me. But while I express a self-locating proposition, that isn't what I communicate or even try to communicate. For if you accept the self-locating proposition that I am expressing, you will thereby take yourself to be sitting, and that's not what I am trying to communicate. So there is a difference between what I am expressing and what I am communicating: I am expressing a self-locating proposition that I am sitting, but I am communicating the non-self-locating proposition that Alex is sitting. Moreover, the two are closely related. Quite plausibly, in accepting the self-locating proposition that I am sitting, I am also accepting a non-self-locating version of it. It could even be that I express both.
But a similar thing happens with time. I hereby write: Alex is now sitting. In so doing, I express a tensed proposition (apologies for the use of "tensed" for non-linguistic entities), a temporally-locating proposition. But I do not communicate the same proposition to you. For while you might infer that I may still be sitting when you read the message, that's a risky inference of yours, not just what I communicated to you. If you just want to believe what I have informed you of, you will believe something that you may express with words like "Alex was sitting then." So there is a difference between what I expressed and what I communicated.
You might think that the difference is not a difference in kind. After all "Alex was sitting then" itself expresses a temporary proposition because of the past tense "was". But that, I think, is just an artifact of the fact that when you went from my "Alex is now sitting" to your "Alex was sitting then", you didn't just accept something that I communicated. Rather, you took what I communicated and combined it with the fact that my communication temporally precedes your reception of it, a fact you know empirically (but it would not affect my argument if you knew it a priori—it's still a fact over and beyond my communication). Sticking to what I communicate to you, you cannot think more than some proposition like that Alex is sitting at that time (where "that" refers to the time of my utterance).
There are now two options. We could go the presentist route and say that both what I expressed and what I communicated are tensed propositions. On this reading, what I expressed was that Alex is now sitting, but what I communicated was that Alex was, is or will be sitting then. But this doesn't seem to me to be a very attractive theory. For when things go right in communication, I shouldn't be communicating a proposition I didn't express, while to claim that I expressed two tensed propositions, though only one was communicated, seems odd. It makes it sound as if you only half believed what I said.
The superior reading, I think, is that I expressed both a tensed and an untensed proposition, and what I communicated was the untensed one.
These things combine. When I say: "I am sitting now", I express three things: a self-locating tensed proposition that I am sitting now, a non-self-locating tensed proposition that Alex (or that guy/gal) is sitting, and a non-self-locating untensed proposition that Alex is (tenselessly) sitting then. But only the last of these do I communicate when I communicate across a relevantly large time delay. But that's all predicated on the view on which there are de se propositions.