>Two important concepts for describing time are "durations" and "instants"

The standard name for durations in physics are "periods" or 'uppercase T' ('lowercase t' being a point in time), which curiously enough are the inverse of a frequency (or the frequency is the inverse of). A period can also be thought of as an interval [t0,t1] or inequality t0<=T<=t1

> The concept of "absolute time" (or "physical/universal time") refers to these instants, which are unique and precisely represent moments in time, irrespective of concepts like calendars and timezones.

Funnily enough, you mean the opposite. An absolute time physically does not exist, like an absolute distance, there is no kilometer 0. Every measurement is relative to another, in the case of time you might use relative to the birth of (our Lord and saviour) Jesus Christ. But you never have time "irrespective" of something else, and if you do, you are probably referring to a period with an implicit origin. For example if I say a length of 3m, I mean an object whose distance from one end to the other is 3m. And if I say 4 minutes of a song, I mean that the end is 4 minutes after the start, in the same way that a direction might be represented by a 2D vector [1,1] only because we are assuming a relationship to [0,0].

That said, it's clear that you have a lot of knowledge about calendars from a practical software experience of implementing time features in global products, I'm just explaining time from the completely different framework of classical physics, which is of course of little use when trying to figure out whether 6PM in Buenos Aires and 1 PM in 6 months in California will be the same time.