[LEAPSECS] Leap seconds have a larger context than POSIX

[Brooks Harris]

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.

-Brooks