[tz] Java & Rearguard

[Steve Summit]

Paul Eggert wrote:
> Steve Summit wrote:
> > I'm not sure that's an entirely fair challenge, though.
> > Given that (as I understand it) Java and ICU/CLDR use tt_isdst
> > to decide whether to display their equivalents of "GMT" or "IST",
> > I don't think they *can* get the right answer near 1970
>
> Yes, Ireland in 1970 is an "unfair" challenge. That was its point. It was 
> intended to illustrate the inadequacy of the current CLDR/Java model to 
> represent real-world aspects of civil timekeeping.
>
> > tzdb changed its mind about the mapping at that point.
>
> I'm not sure what you mean by "mapping",

Java/CLDR's mapping wants to be dst=0 -> GMT, dst=1 -> IST, which
is of course incompatible with tzdb's current mapping.  But if
Java/CLDR were to somehow change to dst=0 -> IST, dst=1 -> GMT,
to match tzdb's current mapping, it would then start getting
the wrong answer before 1970, because for dates before 1970
(in 2019a, at least), tzdb uses dst=0 -> GMT, dst=1 -> IST.