sudanglo:PMK. would it be helpful to consider what antaŭ 2 tagoj means. I suppose that strictly any time more than 2 days ago is also antaŭ du tagoj but I don't think that anybody would use it that way. So if today is Wednesday then li alvenis antaŭ du tago would mean that he arrived on Monday.

In the same way, if today is Wednesday, li alvenos post du tagoj would mean that is expected to arrive on Friday.

I found this in the bible in Hoŝea (old testament)
1 Ni iru, ni revenu al la Eternulo; ĉar Li disŝiris, sed Li ankaŭ resanigos nin, Li frapis, kaj Li ankaŭ bandaĝos niajn vundojn. 2 Li revivigos nin post du tagoj; en la tria tago Li restarigos nin, kaj ni vivos antaŭ Li.

You could check this in your English version but the implication is clear

Huh. . .

Well, first off, I think that the equation with "antaŭ" is astute and very helpful. Certainly "antaŭ" is used to mean "ago" in English.

In that case, if "post" is the forward-looking equivalent then it would be used as we would use "in," as in "in two days." That is, if today is Monday, then "in two days" will be Wednesday.

I think that cracks the code.

Glad, you found my comment helpful. I did have one further thought. If you look at the dictionary definitions for temporal use of antaŭ and post, you see that they reference alian tempon.

But du tagoj is a period not a tempo-punkto. So how do antaŭ du tagoj and post du tagoj come to refer to a point in time.

I suggest that this happens because the current day is the implied alia tempo.

So without any other temporal reference these two expressions mean 2 days in the future measured from hodiaŭ or two days in the past measured from hodiaŭ.

If we supposed that post du tagoj referenced the end of the 2 day period as the implied alia tempo, (te. post kiam estos pasintaj du tagoj) then perhaps post du tagoj could mean today's date + 3 rather than today's date +2.

But that doesn't seem to be the actual usage.
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Metsis:So, according to that Bible quote, post requires full two days to pass and only on the third something further can happen.

Well here's what's interesting about that Bible quote. . .

Most English translations translate that as "after two days." For instance, the NIV says, "After two days he will revive us. . ." So two full days pass and then we will be revived.

However, some of the less literal and more interpretive translations take a bit of a different route. Some examples:

"In just a short time he will restore us. . ." (NLT)

"In two or three days he will heal us . . ." (CEV)

"In two or three days he will revive us. . ." (GNT)

What I suspect is that, for scholars today, the ancient Hebrew is slightly unclear. Probably a literal translation is "after two days," but perhaps very much like we are discussing and debating the usage of "post" and its potential ambiguity, I bet the Hebrew has the same quality. And they are not sure if it means "after two days" or "in two days," according to our present way of speaking.

sudanglo:Glad, you found my comment helpful. I did have one further thought. If you look at the dictionary definitions for temporal use of antaŭ and post, you see that they reference alian tempon.

Etc. . .

Indeed. That makes sense.

If just looking at the pure literal sense of the expression, I would that both "antaŭ du tagoj" and "post du tagoj" would mean "after the conclusion of two days." So if it's Monday, then Monday to Tuesday is the first day. Tuesday to Wednesday is the second day. And Thursday is "post du tagoj." Or you can go the other day working backwards using the same template.

But as you say, that doesn't seem to be the actual usage. They seem to be synonymous with how we would use "X days ago" or "in X days."

I think you are trying to find a one-to-one translation between temporal "post" and English. Translate ideas, not words.

Begin with the meanings of antaŭ and post, i.e. something happens before resp. after an event. How these get translated to your native language quite likely depends on the context. At least I found this when I translated the following examples into my native language. I leave translating these to your native language as an exercise.

an event before a past event

• Ni manĝis, antaŭ ol li alvenis.

an event between a past event and the current time

• Antaŭ kelkaj jaroj mi vizitis Seulon.
  
• Post kelkaj jaroj da malliberiĝo oni amnestiis ŝin.

an event between the current time and a future event

• Antaŭ la sekva semajno mi ne povas partopreni.
  
• Kiel ne venis al vi en la kapon diri, ke post monato oni povas sciiĝi ankoraŭ pli bone.

an event after a future event

• Mi forveturos post monato de hodiaŭ.

Note, that antaŭ and post establish a point in time, while their counterparts ĝis resp. de have a length. Cf. for instance with La rezultoj, kiuj sin montris ĝis hodiaŭ, plifortigas mian kredon and Jam de monato ili ne ricevis sian salajron.

Finally note that you can use the preposition en with time. Example (from a news article describing quarantine regulations in China) Nur po unu homo el ĉiu familio rajtas unufoje en du tagoj iri al vendejoj por aĉeti necesaĵojn.