Re: [art] Predictable Internet Time

[Joe Touch <[e-mail]>]

On 3/28/2017 5:06 PM, Nico Williams wrote:
> ...
>> 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".
That's the trouble - seconds in SI are not seconds defined as 86400/day.
If the units don't match,  then a conversion needs to be included. If
the units aren't known, you can't convert accurately.

>
>> 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.
2017 is 1.4e9 seconds after the Unix epoch. If the Unix and SI "seconds"
unit is off by one part in a (US) billion, then you're off by 1.5 seconds.

>
>> 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.
s/insane/undefined/

That's the problem. I appreciate it would be nice if there were a
definition (as a start), and if the defined unit were SI, but it isn't,
AFAICT. If you have a ref to the contrary, I'd be more than glad to
include it and correct the doc accordingly.

>
>>> 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?
To you and me, perhaps, but there are others on this thread that are
misled by what POSIX is and how easy/hard it is to convert to TAI/UTC...

Joe