Re: [art] Predictable Internet Time

[Joe Touch <[e-mail]>]

On 3/28/2017 3:40 PM, Nico Williams wrote:
> On Tue, Mar 28, 2017 at 02:40:07PM -0700, Joe Touch wrote:
>> On 3/28/2017 10:39 AM, Nico Williams wrote:
>>> On Tue, Jan 03, 2017 at 06:35:03PM +0000, Stewart Bryant wrote:
>>>> Yes, the system should use a leap-second-free constant-duration-seconds time
>>>> for everything. It is only humans that need the variable jumpy version of
>>>> time presented to them, and that is a UI issue.
>>> I don't think that's quite right.
>>>
>>> Unix time is canonically a count of seconds since an epoch and admits no
>>> leap seconds. 
>> Unix time also uses a completely different definition of a "second" than
>> UTC.
> Did I.. say otherwise?
Yes, below....
>
>> 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.

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

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

> There's no need to assess the quality of the local clock or anything of
> the sort.
>
>>> ...
>>> It's all about data typing. 
>> That is certainly one problem, but not the only one.
> 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.

Joe