[LEAPSECS] Leap seconds have a larger context than POSIX

[Martin Burnicki]

Tony Finch wrote:
> The IERS Bulletins C state a value of UTC-TAI "until further notice".

As I understand this, it just means that yet it's not known if and when
another leap second will have to be scheduled.

> 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.

In my opinion this expiration date is very important for those who
evaluate the file, e.g. ntpd.

If ntpd had a leap second file which doesn't contain a leap second after
2016, it didn't know it there really was no other leap second, or if
only the file was too old and hadn't been updated.

In the latter case ntpd wouldn't even know if the TAI/UTC offset derived
from the available leap second file is still valid, or if another leap
second has been inserted in the mean time, and the TAI/UTC offset has
changed.

With the expiration date ntpd can be sure there was no other leap
second, and the TAI/UTC offset is still valid, as long as the current
time is before the expiration date, and the file can be ignored after it
has expired.

If the bulletin C is published once every 6 months, and you have the
bulletin from January, you know for sure if there will be a leap second
at the end of June, or not.

However, only the next bulletin C from next July will tell you if there
will be leap second at the end of December. So in January you can be
sure the information you have won't change until shortly before the end
of December, which is about 11 months.

Even after the next bulletin C and leap second file have been published
in July, things won't change at least up to the expiration date of the
previous file. So you have still about 5 months to deploy the leap
second file from July to your ntpd instances, before anything expire.

One way to propagate a new leap second file is to

- add the new file to the TZ DB some time after tit has been published

- the new file appears in the TZ DB when a new TZ DB versio is released

- after this it appears at the IANA web page, and eventually in software
update packages for your favorite operating system

In summary, it can take weeks or even months until the file is available
everywhere.

So IMO it's very practical that the expiration date is there, and that
you have about 5 months to deploy a new version.


Martin
-- 
Martin Burnicki

Senior Software Engineer

MEINBERG Funkuhren GmbH & Co. KG
Email: martin.burnicki at meinberg.de
Phone: +49 5281 9309-414
Linkedin: https://www.linkedin.com/in/martinburnicki/

Lange Wand 9, 31812 Bad Pyrmont, Germany
Amtsgericht Hannover 17HRA 100322
Geschäftsführer/Managing Directors: Günter Meinberg, Werner Meinberg,
Andre Hartmann, Heiko Gerstung
Websites: https://www.meinberg.de  https://www.meinbergglobal.com
Training: https://www.meinberg.academy