Re: [art] Predictable Internet Time

[Joe Touch <[e-mail]>]

Hi, Nico,


On 3/28/2017 6:58 PM, Nico Williams wrote:
> On Tue, Mar 28, 2017 at 05:15:58PM -0700, Joe Touch 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.
> POSIX does NOT define seconds in terms of days!  It does the opposite:
>
> http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_16
>
>     How any changes to the value of seconds since the Epoch are made to
>     align to a desired relationship with the current actual time is
>     implementation-defined. As represented in seconds since the Epoch,
>     each and every day shall be accounted for by exactly 86400 seconds. 
>
> There is NO reference to solar or sidereal days or any other
> astronomical definition of day.
OK - we agree that it defines "day" = 86400 "seconds".

It should be clear that it also never defines either "day" or "seconds".

"Seconds" can't be SI units unless there's a tiny cesium clock inside my
PC. "day" can't be apparent or mean solar time because it doesn't have a
sextant either.

> Unlike as with days, there is no multiplicity of generally accepted
> definitions of seconds, so there is no support for the idea that this
> text defines seconds in terms of days.
>
> The second sentence defines days in terms of seconds, not the other way
> around.
>
> The first sentence is a bit abstruse, but no matter, because in
> _practice_ on all systems I know of that claim to implement POSIX, or
> which don't but which are Unix or Unix-like systems, seconds since the
> epoch differs from UTC *only* by leap seconds -- just like TAI.
The text only states that the POSIX clock *started* with a known offset
to TAI and that the POSIX clock won't jump with leap seconds.

It never states anything else...

> ...
> POSIX seconds are not defined, indeed, but they can only be taken to be
> SI seconds -- anything else would be sheer madness and, anyways, not
> supported by actual practice.  I know of no POSIX or POSIX-ish system
> where this is not true.

I know of no POSIX system with a cesium atomic clock.

The issue is that POSIX really runs off of whatever local source of time
passage is available, and can (and does) drift from TAI on *each*
system. That drift can't be determined until your system tries to sync
with an external source (e.g., via NTP).

Joe