[tz] Java & Rearguard

Mark Davis ☕️ mark at macchiato.com
Thu May 30 13:50:26 UTC 2019

> the most common (standard) time

I predict that significant libraries and implementations will be on
rearguard forever; if that is not maintained by the TZDB, the it would be
cloned and maintained externally with rearguard modifications. The cost of
doing anything else would be prohibitive.

And there was really no reason whatsoever for changing policies. If we have
a timezone with two separate offsets, one an hour ahead of the other, then
the TZDB can chose to represent them as <0, 1> or as <-1, 0>. It is
needless to have both in the database. For consistency one can simply
decide to have them all be positive, which was the longstanding policy of
this group until recently.

The *name* of the time variants might use "standard" or "winter" or "lětni
čas" or "夏時間", whatever is appropriate to the user's locale. There is no
requirement that any two locales be aligned in what they call the "hour
ahead" variant or the "hour behind" variant. The names attached to the
variant by a given locale is really outside of the scope of the TZDB.

Changing the policy to allow <-1, 0> has no particular practical benefit,
and has just caused compatibility difficulties. You can't just wave a magic
wand and expect longstanding APIs to return different values without
causing problems for users.


And BTW, in most timezones around the world, a majority of the days in the
year are on "daylight" time, so it is the ‘most common one’.

On Thu, May 30, 2019 at 2:04 PM Robert Elz <kre at munnari.oz.au> wrote:

>     Date:        Thu, 30 May 2019 11:54:35 +0100
>     From:        Stephen Colebourne <scolebourne at joda.org>
>     Message-ID:  <
> CACzrW9CXTRW_G07E2Y-b2Q4+Ew5jt8LcFSrzR-YSg0gT-0SCTw at mail.gmail.com>
>   | Finally I'll note that *both* views of the data are sensible and
> reasonable:
>   | - offset-focus: base/standard time in winter, advanced/daylight time
>   | in summer (Java's choice and tzdb's old choice)
> tzdb never made such a choice - it simply didn't have any data that
> happened to have the most common (standard) time advanced ahead of
> the less common offset.
> What are you planning to do when some part of the US (say Illinois, or
> something else on Central time) decides to set their zone backwards by
> an hour for a month in the middle of winter - perhaps the winter olympics
> come to Denver or something, and they decide that being in the same
> timezone for that period has more economic and social wins than being
> an hour off the event times.
> Certainly that might be unlikely, but it certainly is not impossible,
> and you're not likely to get much more than a year's warning if it
> were to happen (at best).   Wouldn't planning for this kind of thing
> now be much more rational than just sticking your heads in the sand
> with a "we didn't consider that and it is too late now" attitude.
> If the old APIs need to be deprecated, and a whole new set invented,
> then so be it - do that - the old ones can be supported, as best they
> can be given they are based upon false assumptions, for a long time,
> but averything should be encouraged to convert to something rational,
> with no in-built assumptions about what is possible or rational wrt
> local time.
>   | - legal-focus: follow government law as to the meaning of
>   | standard/daylight (tzdb's new choice)
> First, standard time is the time that applies *now* - whenever now is.
> If it has a name, distinct from the name that applies to the time at
> some other time of the year, that's fine (but almost irrelevant to
> anything).
>   | Most Java libraries aren't going to change because doing so would
>   | impact compatibility in APIs.The real problem here is how tricky it is
>   | to reverse engineer the old data from the new data.
> I suspect that the real problem is that the current APIs are simply
> inadequate to describe real world time behaviour.   Any assumptions
> made, anywhere, about almost anything, will almost guarantee that.
> kre
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