[tz] [english 100%] Re: OpenJDK/CLDR/ICU/Joda issues with Ireland change

Peter Ilieve peter at aldie.co.uk
Sun Jan 28 12:06:02 UTC 2018

On 28 Jan 2018, at 05:34, Robert Elz <kre at munnari.OZ.AU> wrote:

> From: Meno Hochschild <mhochschild at gmx.de>
> Message-ID: <15c91fab-ca48-d6bd-f961-2b7ce00e7b15 at gmx.de>
> Date: Sat, 27 Jan 2018 21:49:27 +0100
> Subject: Re: [tz] [english 100%] Re: OpenJDK/CLDR/ICU/Joda issues with
>      Ireland change
> | About the wish for the restriction that a set of rule lines with same 
> | name should not have mixed signs of dst offsets, I will try to explain. 
> | The whole thing is about labelling what the zero dst offset stands for. 
> The issue is that it does not (by itself) "stand for" anything.
> This is one of the (invalid) assumptions that people tend to make,
> probably because it has mostly seemed to be universally true in
> (most of) the US, and in Europe, and yes, in Australia, and of
> course in those places that have only ever had one zone offset since
> standardised time was adopted.
> But jurisdictions are free to (without implementing any kind of
> annual time shift, ala summer time or daylight savings) alter their
> timezone (that is, shift their clocks relative to UTC) any time
> they like,  as often as they like, and to anything they like, and can
> alter the name they call their time (assuming that it is called anything
> other than "the time") any time they like as well (either at the same
> time as a change of offset occurs, or just at some random time for
> whatever reason appeals.).
> There is simply no one name, or UTC offset, for "standard" or "alternate"
> time in any timezone - attempting to force one is simply wrong.

This isn’t just some theoretical possibility either. In Europe, with
its reasonably stable rules, Portugal has changed timezone twice within
my memory: in 1992 and 1996. Looking at the tz data, it has changed twice
more in my lifetime: in 1966 and 1976. For the earlier two changes the
clocks changed and the isdst flag didn’t (Portugal didn’t do DST then).
For the later two the clocks didn’t change but the isdst flag did.

As Robert Elz says, almost anything is possible.

		Peter Ilieve

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