[tz] OpenJDK/CLDR/ICU/Joda issues with Ireland change

Guy Harris guy at alum.mit.edu
Sat Jan 27 07:58:43 UTC 2018

On Jan 26, 2018, at 9:34 PM, Clive D.W. Feather <clive at davros.org> wrote:

> Guy Harris said:
>> 1. The ISO C API (localtime()) has a notion of "Daylight Saving Time" being in effect or not in effect, which controls whether tm_isdst is set or not.
> Right.
>> "Daylight Saving Time" presumably refers to turning clocks forward.
> No. That's not stated anywhere in ISO C - it's an assumption you've made.

OK, so what C90 says is

	...Daylight Saving Time, which is a temporary change in the algorithm for determining local time. The local time zone and Daylight Saving Time are implementation-defined.

So, in the Republic of Ireland this still allows "Irish Standard Time" to be considered "Daylight Saving Time", as well as allowing "Greenwich Mean Time" to be considered "Daylight Saving Time", given that the time changes twice a year, rendering both changes "temporary".

(Perhaps the next C standard should just call it "Some Other Time".)

Thus, I *still* don't see a problem with ISO C if tm_isdst is set to 1 for Irish Standard Time, and the conclusion in my item 1 in the message to which you're replying is unchanged.

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