Re: [art] Predictable Internet Time

[Nico Williams <[e-mail]>]

On Tue, Mar 28, 2017 at 04:02:46PM -0700, Joe Touch wrote:
> On 3/28/2017 3:40 PM, Nico Williams wrote:
> > Did I.. say otherwise?
> Yes, below....

You're picking non-nits.

> >> There is no simple conversion between Unix time and UTC. Conversion
> >> requires accurate assessment of the local Unix clock vs SI to determine
> >> a rate conversion as well as knowing the table of leap seconds since the
> >> Unix epoch.
> > You're complicating things.  Forget the local clock.  To convert a time_t
> > value to UTC requires just a list of leap seconds (and some arithmetic).
>
> time_t does not access a SI reference or frankly any other stable reference.

The Open Group man pages I'm looking at don't reference SI, but so
bloody what.  If it says "seconds", it means "seconds".

> If you don't have a stable time unit, accurate conversion isn't possible.

Sure, but POSIX time clearly uses the only definition of seconds that
matters.

Also, time is relative.  These seconds are seconds at mean sea level on
Earth, but they might vary nonetheless.  At some point precision is not
so interesting, but knowing 1490745768 is 2017-03-29T00:02:48Z, that's
kinda important, and that will be true in any local frame of reference,
even in space, far from Earth, and under large accelerations.  Sure, two
such observers won't agree as to current time because of relativistic
effects, but they will agree that 1490745768 is 2017-03-29T00:02:48Z.

> If you do have a stable time unit, you need the ratio of that time unit
> to SI.

It's very safe to assume it's 1.  Anything else would be insane.

> > It's really the main one, IMO.  In any case, smearing seems very wrong:
> > you end up with more problems because you go from roughly two kinds of
> > time (UTC vs TAI-ish) to three kinds of time (UTC vs TAI-ish vs the new
> > thing).  If people had a hard time interoperating with two kinds of
> > time, imagine how it would be with THREE kinds of time.  And if the
> > smearing formula ever needs updating, then we'd be in trouble.  (Earth's
> > rotation normally _slows_ over time, but the 2004 earthquake _sped up_
> > Earth's rotation.  A few big ones and we might need negative leap
> > seconds.  PIT's formula can't predict these events.)
> >
> > Just specify, in each protocol, the use of TAI (x)or UTC.  Done.
>
> I agree.

Isn't that the important bit of information here?

Nico
--