[tz] isdst bug Europe/Dublin (tzdb-2019c)

Paul Eggert eggert at cs.ucla.edu
Mon Dec 16 00:53:06 UTC 2019

On 12/14/19 4:04 PM, Robert Elz wrote:
> It makes no sense (to me) at all to convert TAI into any kind of
> (currently used) calendar type measurements - the calendars we use
> are designed to match astronomical reality

Well, sort of. Astronomers normally match astronomical reality only by using the
technology of 45 BC, because they define a "year" to be the Julian year (31.5576
Ms) when, for example, they estimate the Big Bang as being 13.798 ± 0.037
billion years ago. Paleontologists are similar. Archaelogists too, I expect (at
least for anything more than a few centuries old).

For the Big Bang estimate, the difference between Julian and Gregorian is less
than the confidence interval, so the distinction doesn't matter much. It's like
when someone says "a million years ago", they don't much care whether this means
a million years before 2019 or a million years before 2020.

If we wanted to stick to "astronomical reality" it would not be a good idea to
count Earth days, as the length of the Earth day was only four hours or so back
when the the Earth was formed (at least, according to estimates by Takanori
Sasaki <http://www.iea.usp.br/en/news/when-a-day-lasted-only-four-hours>). We'd
be better off counting Earth years, as the length of Earth's year hasn't changed
as much since then. Of course even then our clocks would go back only 4.5
billion years ago, and would stray a bit from astronomical years for other
relatively-minor reasons (such as, the Sun's mass is constantly declining
slightly, which means the Earth year length is constantly growing slightly).

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