[LEAPSECS] leap seconds in POSIX

[Steve Summit]

Hal Murray wrote:
> Does anybody know of a good writeup of how to fix POSIX to know about
> leap seconds and/or why POSIX hasn't done anything about it yet?

I don't know why POSIX hasn't done anything, other than that
(1) it's an excruciatingly hard problem, made even harder by
(2) the ubiquity and the specific POSIX definition of time_t.

> I assume the basic idea would be something like switch the kernel
> to use TAI rather than UTC

But that seems to end up being a non-starter, because
(1) distributing updated TAI-UTC tables is really hard
(and basically impossible for certain classes of embedded and/or
isolated systems), and (2) everyone knows how to initialize a
system clock to UTC at bootup, but almost no one knows how to do
so for TAI.

The other basic idea is to implement new, proper-UTC-capable
kernel mechanisms and interfaces alongside the time_t ones.
I'm still a big fan of the clock_gettime(CLOCK_UTC) approach
described by e.g. Markus Kuhn at

	https://www.cl.cam.ac.uk/~mgk25/posix-clocks.html

, but it never seems to have caught on.  I went through the
exercise of implementing it myself in a homebrew Linux kernel,
which I used to post a message to this list a few years back
with an honest-to-goodness timestamp of

	Date: Sat, 31 Dec 2016 18:59:60 -0500

Sadly, I've never managed to properly release or publish this
work.  And that sort of gets back to the first part of the
question: Why does there not seem to be much urgency about
"solving" this problem?  Besides the points I mentioned earlier,
I can speculate on a few more:

* Leap seconds are rare, and many people don't care.

* The leap second drought of 1999-2006 rather nastily coincided
  with a gradual change in the computing industry from "nobody's
  clocks are synchronized that well anyway, so a second here or
  there doesn't matter" to the opposite.  (But by the time we
  realized we had a leapsecond problem, it was sort of too late
  to fix it.)

* These days, it seems everybody who has really thought about it
  has concluded that leap seconds are impossible or a really bad
  idea, and is sitting around waiting around for the ITU to
  abolish them, and maybe implementing leap-smeared NTP servers
  to tide us over in the meantime.

Remarkably, though, *Microsoft* of all people seized the bull by
the horns and announced more-or-less proper leapsecond support in
Windows a little while back; it might be instructive to study
that effort for lessons.