[tz] Irish Standard Time vs Irish Summer Time

Paul Eggert eggert at cs.ucla.edu
Sat Jan 20 21:37:29 UTC 2018

Brian Inglis wrote:
> Set when last transition increased offset, clear when last transition reduced
> offset?

An interesting idea, but if taken literally it would depart from the original 
intent of tm_isdst in too many cases. For example, it would mean Africa/Lagos 
would currently have tm_isdst set, because Nigeria's last transition increased 
the UT offset, when it switched from local mean time to standard time in 1919.

I agree with Robert Elz that tm_isdst is vestigial, in the sense that nowadays 
application uses of it are typically unnecessary or even incorrect. The same 
goes for its many counterparts in Java, such as the getRawOffset, getDSTSavings, 
observesDaylightTime, useDaylightTime, and inDaylightTime methods of the 
java.util.TimeZone class. In hindsight, all these interfaces are mistakes: they 
attempt to shoehorn civil-timekeeping practices into boxes that too often don't 
fit, and applications don't really need the interfaces directly anyway. This is 
underscored by the nature of the bug reports reported so far for this problem: 
most of them are for test suites (that is, they're testing vestigial interfaces 
and so are relatively unimportant in and of themselves), and the only 
application-level bug report showed that the application was clearly wrong in 
its use of tm_isdst.

All this being said, tzdb can't simply ignore these vestigial interfaces, as so 
much software uses them (albeit incorrectly).

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