[LEAPSECS] Leap seconds have a larger context than POSIX

[Warner Losh]

On Mon, Feb 3, 2020, 6:01 PM Brooks Harris <brooks at edlmax.com> wrote:

> On 2020-02-03 10:37 AM, Michael Deckers via LEAPSECS wrote:
> >
> >
> >    The 1970 report also contains the proposal that the CIPM should be
> >    responsible for the definition of UTC, and 49 years later, the CGPM
> >    in 2019 seems to have taken on that task with the resolution
> >    [https://www.bipm.org/en/CGPM/db/26/2/] which notably has no
> >    requirement that |UTC - UT1| be bounded.
> >
> >
> I'd like to point out something about the language used in this CGPM
> reference I feel contributes to confusion in some forums.
>
> It says "
> - International Atomic Time (TAI) is a continuous time scale produced by
> the BIPM based on the best realizations of the SI second. ...
> - Coordinated Universal Time (UTC) is a time scale produced by the BIPM
> with the same rate as TAI, but differing from TAI only by an integral
> number of seconds..."
>
> This seems to imply that TAI is an uninterrupted incrementing count of
> SI seconds, and that there is some corresponding form of UTC also
> delineated in uninterrupted incrementing count of SI seconds. This is
> misleading.
>
> ITU-R Recommendation 460 says:
> "TAI ... It is in the form of a continuous scale, e.g. in days, hours,
> minutes and seconds from the origin 1 January 1958"
> and
> "UTC ... corresponds exactly in rate with TAI but differs from it by an
> integer number of seconds."
>
> What is meant here, as most on this list will understand, is that both
> TAI and UTC are to be represented in YMDhms form and that UTC "differs"
> from TAI "by an integer number of seconds" *as represented in YMDhms
> form*. There is no meaningful UTC as an uninterrupted incrementing count
> of SI seconds.
>
> Recommendation 460 itself is slightly misleading in this regard. I think
> it would be better if the UTC statement made it clear that UTC is also
> in the form of YMDhms with its "23:59:60" leap-seconds. This can be
> surmised from other parts of the document, but I don't think its clear
> without careful study.
>
> I've heard many discussions where "uninterrupted incrementing count of
> SI seconds" is confused with the YMDhms representations. I wish the
> BIPM, IERS, CGPM, and ITU-R documents were more clear and used more
> consistent language and descriptions.
>

Yes. I wish it were clearer that TAI time is a regular radio expression of
time. Here regular radix means that it every day has 24 hours and every
hour 60 minutes and every minute has 60 seconds.

UTC has an irregular radix where minutes may have 59 or 61 seconds that are
published in table form or as new rows in the table. All past times have a
set and knowable table. Future times cannot be know past 1 second before
the leap second opportunity after the next announced leap second
opportunity.

At the moment there are only two opportunities to consider, though the
standard allows up to end of every month. This ambiguity has lead to bugs
when leaps were announced more than 3 months in advance. I'd feel better if
there were a commitment to these parameters for some number of years. But
it's all just convention today.

Have I forgotten any of the other details of leap seconds that are more
tribal knowledge than rigorously specified?

Warner


-Brooks
>
>
>
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