[tz] Irish Standard Time vs Irish Summer Time

[Paul Eggert]

John Hawkinson wrote:
> in the world of timezone software, with complex downstream dependencies and embedded devices and mobile devices with weird update strategies that take data but not code, a year is *not* a long period of time. It's not long for tzdata, but it is far shorter for code.

As I understand it, the important problem with negative DST lies not in tzcode 
(it works just fine), nor with libraries that use the binary output of zic (so 
far, no problems have been reported for them, and this makes sense if you look 
at the format of the binary data), nor even with applications that use those 
libraries (so far, just one quite-minor and easily-fixed problem has been 
reported, and it was reported only because I looked carefully through perhaps 
the most tzcode-intensive application on the planet).

No, the main problem with negative DST offsets appears to lie in other tzdata 
consumers, which assume that DST offsets must be positive. Until 2018a the zic 
man page did not document that negative DST offsets were allowed in zic input, 
and developers of some other software missed the possibility of negative DST 
offsets even though tzcode has supported them for decades.

We've run into similar problems in the past with tzcode directly. That is, we've 
changed tzcode to extend the format of zic input, and then we've waited a while 
before using these extensions in tzdata. This wait gave downstream developers 
time to upgrade their zic implementations to add support for these extensions.

Given that negative DST wasn't clearly documented until 2018a, I also suggest 
that we extend a sizable grace period to downstream code, as it will take some 
time for people to update that code and get updated versions into production. 
During that transition period, we can provide options so that downstream users 
can use the full data, or a bowdlerized version of the data that avoids negative 
DST but is otherwise as close to the original as is practical. This can all wait 
until after 2018c is announced.