On 2017-02-01 07:39 AM, Steve Summit wrote:
Brooks Harris wrote:
On 2017-01-31 08:21 PM, Steve Summit wrote:
I feel like I should apologize for my earlier contribution to it,
which presented a nice-looking, persuasive-sounding argument
which now looks an awful lot like it's... wrong.
Not at all. Its an informed contribution. And I think it's not "wrong".
At least not yet. I think its "right", so far. :-)
On further reflection, I think we're all right. For every
let's-look-at-the-arithmetic argument that suggests we should
use the "new" offset during the leap second, I can come up with
one which suggests the opposite. (Basically it depends on
whether you come at the leap second "from below" or "from above".
I'll send the longwinded details in a separate message, if anyone
actually cares.) So I'm right, and you're right, and Warner's
right, and Steve Allen is especially right in his assertion that
it's just inherently, fundamentally ambiguous.
Yes, well its that "fundamentally ambiguous" part that none of us can
stand and keeps us obsessed with the LEAPSECS discussion, I guess.
I continue to believe TAI-UTC updates after the Leap Second, at the UTC
YMDhms midnight rollover. Digging deeper, and once again reading Rec 460:
https://www.itu.int/dms_pubrec/itu-r/rec/tf/R-REC-TF.460-6-200202-I!!MSW-E.doc
Section C Coordinated universal time (UTC) says "UTC is the time-scale
maintained by the BIPM, with assistance from the IERS..."
This says BIPM is the authority that maintains the UTC timescale,
doesn't it?
Section E DTAI says "... The TAI - UTC values are published in the BIPM
Circular T ...".
OK, so, better look more carefully at Circular T.
CIRCULAR T 348 ISSN 1143-1393
2017 JANUARY 10, 10h UTC
ftp://ftp2.bipm.org/pub/tai//Circular-T/cirthtm/cirt.348.html
And there we find - "1 - Difference between UTC and its local
realizations UTC(k) and corresponding uncertainties. From 2015 July 1,
0h UTC, TAI-UTC = 36 s. From 2017 January 1, 0h UTC, TAI-UTC = 37 s."
I think this clearly says TAI-UTC updates on 2017 January 1, 0h UTC, the
midnight rollover. This time-point is *after* the Leap Second, isn't it?
And its authoritatively stated by BIPM.
Meantime, IERS Bulletin C says the same thing - "from 2017 January 1, 0h
UTC, until further notice : UTC-TAI = -37 s"
https://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat
I'm not understanding how this can be interpreted any other way. There's
nothing ambiguous about it to my reading, and I've had this conversation
with other good engineers that concur with that reading. It may not be
convenient for some methods of converting TAI seconds to UTC YMDhms
representation, but there it is, in black and white, near as I can see.
It also lines up with all the other IERS Leap Second history data products.
You can't just ignore this because its more convenient. Of course you
can adjust it internally in a specific implementation as required or
convenient, in fact you probably need to in some way or other. But the
data supplied by BIPM and IERS needs to be interpreted consistently, right?
-Brooks
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