[LEAPSECS] BBC radio Crowd Science

[Warner Losh]

On Tue, Jan 31, 2017 at 12:41 PM, Brooks Harris <brooks at edlmax.com> wrote:
> 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.

All the specifications says is that positive leap seconds cause the
minute to have 61s.

>>>
>>> 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?

It increments just prior to the positive leap second. It doesn't
matter how you get the information. That's when the offset needs to
change for the irregular radix math to work.

Or put less authoritatively: why would it matter how you get the
information? A second is either a leap second or it isn't, right? And
the property of a second that makes it a leap second in UTC is that's
when the offset changes against TAI.

We know that

UTC = TAI + UTC_OFFSET(TAI)

Since UTC is an irregular radix, the math is a little complicated, but
the only way for the above to work out is if UTC_OFFSET(TAI) changes
at the start of the leap second. So going from 36 to 37, the way you
get :60 is by subtracting 37 from TAI instead of 36.

>>
>> 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.

It's the only way to encode one number such that the irregular radix
math works out. And treating a positive leap second 'special' that
needs an extra decrement is isomorphic to changing the delta at the
start of the second.

> 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?

I think that the standard and standard math concepts of irregular
radix would dictate all that. How you encode your internal tables is
up to you, so long as they are isomorphic to what I'm saying.

Warner

>>
>>
>>> 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
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>
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