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

Robert Elz kre at munnari.OZ.AU
Sun Dec 15 00:04:04 UTC 2019

The problem we have in the current discussion, is that once again,
we're trying to over simplify time - make time work the way we think
it should work, rather than the way it does.

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 (the various rotations of
the earth) and those things just don't take constant periods, even
though they're close.   But that's what we have decided we want from
our calendar system - we want it to alwqays be summer in January, and
winter in July (those unfortunate enough to be in the wrong hemisphere
can adapt as needed), and TAI (alone, withough some kinds of
corrections) simply does not provide that.   If you want some kind of
calendar for TAI you'd be better to convert it to stardates or something.

Certainly I don't think it makes any rational sense to have different
length "years" (regular years and leap years, with a bizarre rule about
which years are which) in TAI - though it is certainly sensible to have a
unit bigger than seconds in which to measure lengthy periods (though I doubt
anything approximating a year is really big enough - TAI is useful for
measuring short durations (something we would conventionally count in
tiny fractions of a second up to hours), and very long ones (millions of
years, and longer), but not so much for anything in between).

What we really need to do is recognise that there is not (rather should
not be) just one definition (one unit) "a second" - there are two entirely
different things we use commonly, which just happen to be so close together
that we have tried to force them to be the same thing.

One is the thing TAI uses (and UTC with leap second corrections) - which is
a period defined by something that we believe to be fixed - always the exact
same duration, and so can be used for comparing durations from measurements
collected separately,

The other is 1/84600 of a day (a bizarre choice, but never mind) where a
day is defined according to the (varying) rotations of the earth, and hence
the second is not a fixed length, but a variable one.   That's the POSIX
second (time_t) value, but is also how humans have measured time ever since
we started doing it - initially it wasn't understood that the length of a
day wasn't constant (as it so nearly is) but 60 seconds a minute, 60
minutes an hour, 24 hours a day, (ie: 86400 seconds a day) is what has been
used for a long time now, (with the more precise measurements appearing as
we gained the ability to measure them, sundials to be able to measure house,
hourglasses to meaure minutes, and fractions thereof somtimes, ...)

We really should be using two different names for these two different things,
rather than trying to pretend that a "second" can be defined which suits
both purposes.

But until a large enough fraction of the population need to routinely
deal with the differences, I don't see that happening.   Certainly it
doesn't help to continually try and force one another to adopt only one
way of measuring time, as there is (as long as we keep anything like our
current calendars) no way to make that work simply - nothing anyone would
be prepared to attempt to implement - that is even if one can accept that
any current calendar calculations are even close to simple.


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