[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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