[tz] Java & Rearguard

[Robert Elz]

    Date:        Wed, 5 Jun 2019 11:51:25 +0100
    From:        Stephen Colebourne <scolebourne at joda.org>
    Message-ID:  <CACzrW9C+bnc0waSbcaVisHG-cJqQ2q8pMTvT1a8aL4uJjC9uwg at mail.gmail.com>


  | The assumption is that daylight=summer/advanced, and this goes a long
  | way down the stack, notably into CLDR.

What are you planning to do when some jurisdiction does summer/retarted ?

And before anyone claims that's absurd, it isn't really.   Queensland in
AU has never really done summer time (they tried it briefly, once, I think).
I suspect that it is a combination of the lifestyle in the areas that really
matter (the south east corner) along with that being moderately east in
the +1000 time band - that is, they already have natural negative summer
time.   In summer, which is also monsoon (cyclone) season, it tends to
get dark(ish) early (caused as much by thick clouds as sun position) with
plenty of rain around - making it daylight later would not help a lot.
On the other hand, it also gets light early - and is warm and fine early,
so lots of people do (or did when I lived in that area) use the early
morning hours for what people in other places tend to do in the evenings
(recreationally, going out to eat, movies, etc, doesn't need daylight...)

That is, it is commmon for people to wake up around 5 (when the sun has
been up for a while) and go fishing, surfing, play a round of golf, or
whatever, and after that, go to work - rather than wanting to do any
of those things in the evenings when it is less likely to be practical
because of the weather.

In that environment, moving more sunlight period earlier into the morning
(get the dew off the greens, ...) isn't an absurd idea.

I doubt it will happen, but who knows?    If you are expecting ("assuming")
it can never happen, then you could be in for a lot of problems.

Note, that the strategy of making the earlier time be "standard" time with
tm_isdst 0 in this scenario would have standard time in summer, and winter
time (tm_idst === 1) in winter.   Nothing like anyone would naturally
expect things to be.    And of course even that assume that there is just
one switch - it would be less needed in mid-summer when it gets light
quite early anyway, but of more benefit in spring/autumn - but most likely
with different amounts of time shift for each of those periods (I can
imagine 30 mins in Spring (October) and an hour in Autumn (March - it
fits better).

However long you have been making assumptions about how time works, it
really is time (!?) to stop, and phase out everything - all that is possible
is to convert a UTC timestamp into the local time for the zone, and given
a zone offset, convert local time back to a UTC time (the zone offset is
needed, a "is xxx" boolean is simply not enough).

kre