[LEAPSECS] Bulletin C and all that

[Brooks Harris]

On 2015-01-25 08:15 PM, G Ashton wrote:
> Rob Seaman wrote:
>
>> Interesting if UTC can indeed be said to have implemented two different
> mechanisms whose entire point was to keep Universal Time synchronized with
> Mean Solar Time.
>
> The point of the multiple mechanisms was to keep UTC close to UT which is
> mean solar time at Greenwich, or wherever zero degrees longitude is deemed
> to be.
>
> Rob Seaman also wrote:
>> Leap seconds are introduced at midnight UTC, not when TAI modulo 86400
> equals zero.
>
>   I would think that midnight UTC is the instant when the UTC time becomes
> 00:00:00. I would call the introduced second the one that began at 11:59:60
> and ended one second later at 00:00:00.

Yes. That's what ITU-R  TF.460-6 says. Its made very clear in Annex 3.

-Brooks

>
> Gerard Ashton
>
> -----Original Message-----
> From: LEAPSECS [mailto:leapsecs-bounces at leapsecond.com] On Behalf Of Rob
> Seaman
> Sent: Sunday, January 25, 2015 19:26
> To: Leap Second Discussion List
> Subject: Re: [LEAPSECS] Bulletin C and all that
>
> On Jan 25, 2015, at 1:03 PM, Stephen Scott <stephenscott at videotron.ca>
> wrote:
>
>> Since UTC is defined by the IERS before 1972-01-01 "beginning_of_utc" is
> not appropriate.
>> This is the beginning of integer leap seconds, not UTC.
> Contributors to this list can always count on prompt fact checking ;-)  That
> said, the IERS came later than that, didn't it?
>
> Interesting if UTC can indeed be said to have implemented two different
> mechanisms whose entire point was to keep Universal Time synchronized with
> Mean Solar Time.
>
>> How about "leap_second_epoch" or if the term epoch is undesirable
> "leap_seconds_origin" labelled as "leap00"
>
> Ok, I'll re-index to leap0 and have a new cname called origin.leapsec.com.
> How's that?
>
>
> On Jan 25, 2015, at 2:01 PM, Brooks Harris <brooks at edlmax.com> wrote:
>
>> TAI is often also represented as a date-time but there is rarely a clear
> distinction made about what it means.
>
> TAI is most naturally expressed as an unending tally of SI-seconds.  UTC as
> a sexigesimal fraction of a solar (synodic) day.  It is conversion between
> the two concepts that get us into trouble.  This DNS scheme might provide a
> small step toward letting them live together in greater harmony, and the
> tzdist standard a larger step addressing additional use cases.
>
>> And, Rob, what, exactly, does "1972  1" mean in your Leap Seconds table?
> 1972-01-01T00:00:00 (TAI) or 1972-01-01T00:00:00Z (UTC)?
>
> As PHK has observed, the essential concept of the IPv4 DNS leap second
> coding is to express the equivalent of Bulletin C.  I have always
> interpreted the independent variable of the IERS table as UTC.  Leap seconds
> are introduced at midnight UTC, not when TAI modulo 86400 equals zero.
>
> Rob
>
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