[LEAPSECS] Time math libraries, UTC to TAI

[Brooks Harris]

On 2016-12-30 12:56 PM, Stephen Scott wrote:
> NOT "unintentional"
>
> -S
>
>
> On 2016-12-30 11:37, Brooks Harris wrote:
>>
>> In SMPTE standards parlance the first sentence is normative, but the 
>> "Note" is informative. The intention of the note is to inform 
>> implementers that the intention for SMPTE purposes is to interpret 
>> the "PTP Epoch" as integral seconds before 1972-01-01T00:00:00Z (UTC). 
>
>> Unfortunately, and probably unintentionally, the text leaves some 
>> ambiguity because the IEEE 1588/PTP states - "... which is 31 
>> December 1969 23:59:51.999918 UTC" while the SMPTE "note" says, and 
>> the intention is it be, 1969-12-31T23:59:50 (UTC). 
> Having been a prime instigator of that note, it was very deliberate 
> and not unintentional. It says nothing about UTC prior to 1972. It 
> makes clear the relationship between TAI and UTC at 
> 1972-01-01T00:00:00 (UTC) so that the reader is not misled by the 
> ambiguity prior to that date that might be caused by statements in 
> IEEE 1588-2008.
> -S
Right, I was there, and yes, the point it should be


>> Having been involved in these discussions I know the intention is the 
>> latter. The words in a standard matter.
>>
>> One thing for sure - if we can't agree what a particular timescale's 
>> origin, or "epoch", means and its exact relationship to 1972-01-01 
>> 00:00:10 (TAI) = 1972-01-01T00:00:00 (UTC) and we don't implement 
>> them consistently, there won't be interoperability no matter how 
>> exacting all the other details of the counting schemes may be.
>>
>> -Brooks
>>
>>> The choice of TAI - 10 would put the origin of the time scale at the
>>> intersection of the grey vertical line and the green horizontal line
>>> seen in the second plot on
>>> http://www.ucolick.org/~sla/leapsecs/amsci.html
>>> And for any system using that time scale that puts the origin at
>>> 1.999918 SI seconds after the (BIH average of the) radio broadcast
>>> time signals said it was 1970-01-01T00:00:00.
>>>
>>> But all of this is pedantic and moot for any currently operational
>>> system because it was not operational then.  Only a few things like
>>> astrometry of solar system bodies and spindown of a handful of pulsars
>>> have data and models with enough precision to discern this.
>>>
>>> Everything and everyone else can reasonably assert that they do not
>>> have to care and that every day has always had 86400 seconds of
>>> duration equal to what they are currently ticking.
>>
>>> -- 
>>> Steve Allen<sla at ucolick.org>               WGS-84 (GPS)
>>> UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165 Lat  
>>> +36.99855
>>> 1156 High Street               Voice: +1 831 459 3046 Lng -122.06015
>>> Santa Cruz, CA 95064http://www.ucolick.org/~sla/    Hgt +250 m
>>> _______________________________________________
>>> LEAPSECS mailing list
>>> LEAPSECS at leapsecond.com
>>> https://pairlist6.pair.net/mailman/listinfo/leapsecs
>>>
>>>
>>
>> _______________________________________________
>> LEAPSECS mailing list
>> LEAPSECS at leapsecond.com
>> https://pairlist6.pair.net/mailman/listinfo/leapsecs
>>
>
>
> ---
> This email has been checked for viruses by Avast antivirus software.
> https://www.avast.com/antivirus
>
> _______________________________________________
> LEAPSECS mailing list
> LEAPSECS at leapsecond.com
> https://pairlist6.pair.net/mailman/listinfo/leapsecs
>
>