[tz] C++20 time library

[Paul Eggert]

On 9/21/21 9:29 AM, Howard Hinnant wrote:

> Agreed that the link part of the database may be zero-sized on some platforms.

That's fine, but could a note or footnote be added to that effect? For 
example, the text should make it clear that even given the same tzdb 
version, the same entry could be a Zone on one platform and a Link on 
another.

> This part of the library is specified to be interoperable with Unix Time (http://eel.is/c++draft/time.clock.system#overview-1).  Thus leap seconds are excluded by design.

Ah, OK. Unfortunately that's not obvious from the text, which has an 
extensive discussion of leap seconds.

I suggest adding some text that makes it clear that the leap second 
interfaces are only for interrogating tzdb, and that leap seconds are 
not used by the conversion functions. For example, it should be clear 
what conversion functions do when given a tzdb entry that has leap 
seconds - do they ignore the leap seconds, or report an error, or what?