[tz] [english 100%] Re: Irish Standard Time vs Irish Summer Time
Paul Eggert
eggert at cs.ucla.edu
Fri Jan 19 19:31:35 UTC 2018
On 01/18/2018 01:36 PM, Meno Hochschild wrote:
> I also suspect that this is another problem (not yet tested): Java
> would start with v2018a to report Irish Summer Time (IST) in winter
> and GMT in summer when it comes to formatting the Irish zone
> Europe/Dublin.
Yes, it sounds quite plausible that Java/OpenJDK would somehow translate
"IST" to "Irish Summer Time" in Europe/Dublin. This assumption has been
incorrect for quite some time, as for many years tzdata's Europe/Dublin
has used "IST" to stand for "Irish Standard Time" for some timestamps
(notably, between 1968 and 1971). Unfortunately the tzdata format does
not allow this assumption to be stated in the data; it's only in
comments. If the Java/OpenJDK model does not allow the same abbreviation
("IST" in this case) to mean different things at different times in the
same location, that would be a shortcoming in the model regardless of
whether the Irish data moves to timestamps with negative DST offsets.
Java/OpenJDK is a different project from tzcode, and it has always
supported a superset-of-a-subset of what tzcode supports. For example,
Java (at least, Oracle's version of Java) does not support POSIX-format
TZ settings; on the other hand, Java supports long names like "Pacific
Standard Time" that tzcode does not. With that in mind, it would be
understandable if Java/OpenJDK decides not to support negative DST
offsets; it would be just one more thing in the list of things that
tzcode has but Java lacks. If that happens, it should be possible for
Java-based readers of tzdata source files to automatically translate
negative DST offsets into positive ones (switching abbreviations of
course), as I think Stephen Colebourne suggested. It's also possible
that all things considered it would be better if Java supported negative
DST offsets -- but that's not our call to make.
We can give the Java folks some time to prepare for this by backing out
the Irish changes for now. We can bring the changes back in the
not-too-distant future, and in the meantime the Java folks can test
their fixes on 2018b.
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