[tz] Java & Rearguard

Lester Caine lester at lsces.uk
Sat Jun 8 08:47:04 UTC 2019

On 08/06/2019 01:32, Steve Summit wrote:
>>> Java has never provided accurate time zone names for historic times.
>>> I doubt any system does
>> Quite true, in large part because accurate time zone names do not exist
>> for most historic times. That being said, there's no doubt that for
>> Ireland in 1970, tzdb gets it right whereas Java gets it wrong.
> I'm not sure that's an entirely fair challenge, though.
> Given that (as I understand it) Java and ICU/CLDR use tt_isdst
> to decide whether to display their equivalents of "GMT" or "IST",
> I don't think they*can*  get the right answer near 1970, because
> tzdb changed its mind about the mapping at that point.

I'll return to my now long in the tooth complaint that a major part of 
the problem IS that the current system does only cater for current time 
systems. That NAMES are only documented for a very small percentage of 
the world prior to 1970 is in part due to the fact that those areas did 
not have any need for a chronometer that did more than dawn, mid day and 
dusk. But there is a growing archive of material that does have accurate 
historic records and accurate changes to time setting that means that 
'midday' does not correspond with the sun directly over head. The base 
clock IS still the sun, and ALL that this daylight saving debate is 
doing is recognising the fact that in addition to the 24 hour cycle, 
there is a year cycle where for 6 months of the year dawn and dusk are 
closer together and a shift of 'midday' gives a supposedly better spread 
of light during the day. And so for one half of the planet the 'daylight 
saving time' flag is the wrong way up anyway.

There are still a few fixed points that politics can't change even if 
they want to, and 'local mean time' along with dawn and dusk are fairly 
safe ones? So ignoring any 'international system' which is only a recent 
invention, but from a time a little longer ago than 1970, we have a base 
which gives midday, and 'day' longer/shorter than night. CURRENTLY one 
can only rely on that prior to 1970 using the current rules, while for 
the large areas that we do have accurate 'standard time' back into the 
1800's there is not dispute over just what offset was applied to that 
base and the 'flag' that is missing is one of 'standard time' rather 
than 'local mean time', and 'standard time' should only apply where it 
IS valid data not where some other set of data is being used prior to 
1970 just for convenience. And Ireland is a case where pre-1970 data is 
different to the set of data currently being used!

Lester Caine - G8HFL
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