[LEAPSECS] Leap seconds have a larger context than POSIX

[Michael Deckers]

    On 2020-02-04 13:44, Tony Finch wrote:


> The IERS Bulletins C state a value of UTC-TAI "until further notice".
>
> However the machine-readable files from IERS and NIST give an expiry date
> of a few days less than 6 months after the announced (lack of) leap
> second, or a bit more than 11 months after the latest Bulletin C.
> Is this expiry date reliable or just advisory? History suggests it's
> reliable, but the standards do not.
>
> It's unclear to me what governs the frequency of announcements or their
> validity period, i.e. where are current practices documented? what is the
> process for changing them? how will we know if a change is planned? and so
> on. This is all about how much we can assume that the IERS will continue
> to operate leap seconds as they have for nearly 50 years, or whether they
> will make use of the much weaker guarantees given by TF.460, or (wishful
> thinking) whether they can schedule leap seconds further in the future.



    The IERS (and BIH) policy to use only the primary
    choices for the insertions of leap seconds is only
    guaranteed in the text of Bulletin C -- if LOD
    increases sufficiently, that text will have to
    change.

    There is a similar situation for Bulletins D -- each
    of them announces when the next one is expected to
    be issued. But even nowadays these predictions of
    UT1 - UTC are not very reliable, and they often err
    on the "wrong" side (DUT1 changes earlier than
    predicted).

    For instance, Bulletins D139, D134, and D129 each
    came earlier than predicted by the preceding Bulletin D;
    Bulletin D129 (of 2016-04-15) was even significantly earlier
    (45 d) than predicted by Bulletin D128 (of 2016-02-19).

    Michael Deckers.