[LEAPSECS] BBC radio Crowd Science

[Warner Losh]

On Tue, Jan 31, 2017 at 12:57 PM, Brooks Harris <brooks at edlmax.com> wrote:
> On 2017-01-31 02:50 PM, GERRY ASHTON wrote:
>>>
>>> ...On January 31, 2017 at 7:08 PM Steve Allen <sla at ucolick.org> wrote in
>>> part:
>>> I prefer to think of a leap second as being truly intercalary.
>>> It is saying to atomic clock "It's not tomorrow yet, wait a second."
>>> It is between one calendar day of UTC and the next calendar day of UTC.
>>> It belongs to neither of them...
>>
>> UTC is civil time. Civil time is used to express deadlines. Most deadlines
>> fall at the end of a calendar day, so which day the 61st second falls in
>> will only affect a few time zones, but deadlines may fall on other hour
>> boundaries. So it is necessary to know which hour the 61st second belongs
>> to, and I believe it belongs to the hour that is about to end.
>
> I don't think anyone disagrees that the Leap Second is the last second of a
> day, "pushing" the midnight roll-over. That's clear from Rec 460. But when
> the TAI-UTC value increments is an important question.

It increments in such a way that the math for conversion works between
the two. That's the start of the leap second. One might informally say
that the increment is at the start of the day as a convenient
shorthand, but if that short hand results in wrong answers, that
shorthand is not what should be used to compute the answer.

Warner