[LEAPSECS] Leap seconds have a larger context than POSIX

[Brooks Harris]

On 2020-02-04 2:54 AM, Warner Losh wrote:
>
> Have I forgotten any of the other details of leap seconds that are 
> more tribal knowledge than rigorously specified?
>
> Warner
>
>
I think another unclear topic is when the value of DTAI ("The value of 
the difference TAI – UTC") is updated. This is not clearly stated 
anywhere and I've heard many discussions about it, including here on 
this list. The question is if DTAI is updated simultaneous with the 
leap-second of after the leap-second. Rec 460 does not state this. The 
answer is evident only by *implication* in Bulletin C and other 
publications. Bulletin C 52, for example, says " UTC TIME STEP on the 
1st of January 2017", and shows:
from 2015 July 1, 0h UTC, to 2017 January 1 0h UTC : UTC-TAI = - 36s
from 2017 January 1, 0h UTC, until further notice : UTC-TAI = - 37s

Bulletin C does not say or indicate the value will change with the 
leap-second, but immediately after the leap-second, simultaneous with 
the following midnight. Other publications, such as 
Leap_Second_History.dat show this same relationship. But no prose says 
"the value *shall* update *after* the leap-second".

Curiously Bulletin C does not call this "DTAI" as Rec 460 does, and 
gives the value in the opposite sign (Rec 460 calls it "TAI – UTC" and 
Bulletin C calls it "UTC-TAI"). This is an example of the inconsistency 
in the use of terms and nomenclature in the relevant documents at ITU-R, 
BIPM, and IERS, which does not help clarity.

Find is a copy of Rec 460 I've modified at:
http://edlmax.com/Common_Calendar/R-REC-TF.460-6-200202-I!!MSW-E_BEH_V1_2019-11_15.doc

I've added minimal prose changes (red) and an addition to ANNEX 3, 
FIGURE 3 to clarify these two issues, 1) that TAI and UTC are to be in 
the YMDhms form, and 2) that DTAI updates after the leap-second. I feel 
if Rec 460 had these small changes it would make the use of leap-seconds 
much more clear and save many hours of discussion and angst. It took me 
a long time to have confidence in my understanding when I first 
encountered the leap-second some years ago.

-Brooks