[LEAPSECS] BBC radio Crowd Science

[Warner Losh]

On Tue, Jan 31, 2017 at 11:58 AM, Brooks Harris <brooks at edlmax.com> wrote:
> On 2017-01-31 12:33 PM, Warner Losh wrote:
>>
>> On Tue, Jan 31, 2017 at 7:54 AM, Steve Summit <scs+ls at eskimo.com> wrote:
>>>
>>> Tom Van Baak and Michael Decker wrote:
>>>>>>
>>>>>> 2017-01-01T00:00:36.5 - 36 s = 2016-12-31T23:59:60.5
>>>>>
>>>>> What kind of arithmetic is that?
>>>
>>> I think it ends up being roughly the same kind of arithmetic
>>> that tells you that the 60th day of the year is March 1.
>>> Or maybe February 29.
>>
>> Maybe he's referring to the fact that the offset is 37s, not 36s. The
>> offset changes AT THE START OF THE LEAP SECOND.
>
> OK, now here's something I've been worrying about for a long time. Everyone
> on LEAPSECS, and seemingly everywhere else in the literature, are *sure*
> they know exactly what UTC with Leap Seconds is. Yet the specifications are
> unclear, as we've been discussing.
>
> Here you are saying "The (TAI-UTC) offset changes AT THE START OF THE LEAP
> SECOND. " That is in direct conflict with my best understanding of it. I'd
> say "The (TAI-UTC) offset changes immediately AFTER the Leap Second, at the
> midnight roll-over to the first second of the next month." (See other email
> with my explanation and demonstration code).

That code isn't doing what you think it is, at least imho. By knowing
it's a leap second and adding 1, you've just made a complicated
adjustment that would be unnecessary if the offset changes at the
start of the leap second. Effectively you've "corrected" knowing it's
a leap second by changing the answer by one. That's exactly the same
as saying the offset is one greater one second earlier and eliminating
the special case. In both cases, you have to know that the last minute
has 61 seconds.

> So, this is obviously a huge interoperablity issue. It has ramifications
> through many aspects of timekeeping manipulations.

I don't think so. It's all about how you do the math and the final
answer. My interpretation leads to simpler math.

> Ah, so who's right?

IMHO, if you do the math out long hand, you'll see I'm right. See
other mail where I walk through it (though perhaps in a difficult to
follow way due to the limits of email).

Warner