[LEAPSECS] BBC radio Crowd Science

[Brooks Harris]

On 2017-01-31 02:04 PM, Warner Losh wrote:
> 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.
Yeah, but I think that's what the specification say.
>> 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.
I think its about how you *receive* the information too, not just the 
YMDhms (UTC) result. What information is supplied by NTP or GPS? You've 
got to know exactly when to apply the Leap Second. In the case of a 
hypothetic Leap Second aware timestamp you need the metadata: TAI-UTC 
and one of IsLeapSecond or IsLeapSecondMinute (like a "61" flag) or 
IsLeapSecondDay. But the question is, on which timestamp does TAI-UTC 
increment?
> My interpretation leads to simpler math.
Simpler math for what purpose? A bit simpler to calculate YMDhms (UTC) 
maybe, I'd have to work it through. Of course you can treat it anyway 
you like internally as long as two applications wind up with *exactly* 
the same results. But they have to agree when TAI-UTC updates, at least 
as far as any interchange mechanism in concerned, right?
>
>> 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
> _______________________________________________
> LEAPSECS mailing list
> LEAPSECS at leapsecond.com
> https://pairlist6.pair.net/mailman/listinfo/leapsecs
>
>