[tz] Austin Group POSIX TZ changes and Time Zone Taxonomy

Robert Elz kre at munnari.OZ.AU
Thu Feb 23 18:03:19 UTC 2023

    Date:        Thu, 23 Feb 2023 08:12:57 -0700
    From:        Brian Inglis via tz <tz at iana.org>
    Message-ID:  <07fb0dba-622e-4d46-7f0f-7618a014621e at Shaw.ca>

  | I see problems with their terminology,

I don't see a lot of point arguing about terminology, if we can get
them to handle the concepts properly, that's enough.

  | They only allow Area/Location,

No, that is not what the text says.   It does say "Typically these take
the form Area/Location as in the IANA timezone database." but that is not
"only allow".   The original proposal assumed that was all that was needed,
but not the text that was eventually adopted.

  | They do not take account of the fact that a system may not use UTC,

If it is a POSIX system, modified (as in no leap seconds) UTC is what it
uses.   If it isn't a POSIX system, then what the POSIX standard says is

  | may have to accomodate that, and that the zone identifier may 
  | have three levels below that.

The number of levels is immaterial.   The spec allows anything which is
does not start with a ':' (which remains implementation defined) and
which is not in the form of an old style POSIX TZ string to be a TZ
identifier.   Anything.

  | They also need to know that their model is inadequate and four
  | transitions a year between two offsets are now common in many zones,

That's (partly) why the old style TZ string was (finally) accepted as
inadequate.   There are no limitations on the number of transitions in
the new form.

There still are issues with the definition (and setting, and use) of
tm_isdst - but those are largely inherited from C, and really need to
be fixed there (first).

  | Their wording should reflect that the offset changes are not necessarily
  | binary or really anything to do with DST,

That would be nice, but there's only so much that can be reasonably
expected to be accomplished.

How local time works is burned into many people's brains, and they
simply cannot contemplate anything different as possible.   They *know*
how it works.   End of discussion.

  | > tm_gmtoff and tm_zone - do you know what's going on there?
  | Added in	https://www.austingroupbugs.net/view.php?id=1533

Ah yes, thanks, saved me looking it up.

  | See also	https://www.austingroupbugs.net/view.php?id=1613

Only if you're really bored, that one is kind of obvious, and meaningless.

I will also take this opportunity to correct an error I made in my
previous message - issue 1619 (which has the TZ changes) isn't one of
mine, and is entirely about that - what I was thinking of was 1614
(just change the number at the end of one of the URLs above) which is
one of mine, and during the discussions of which (still ongoing, though
that one I thought would have been the simplest one of the 1612/1613/1614
trilogy) the TZ issue arose, and led to 1619.


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