[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:
>>
>> This is the interpretation used in the new SMPTE (Society of Motion 
>> Pictures and Television Engineers) ST 2059-2 and ST 2059-1 standards 
>> for synchronization over network systems. It is based on IEEE 
>> 1588/PTP. It states:
>>
>> ---------------
>> 6 SMPTE Epoch and Signal Alignment
>> The SMPTE Epoch shall be 1970-01-01T00:00:00TAI, which is the same as 
>> the PTP Epoch specified in IEEE
>> Standard 1588-2008.
>>
>> Note: The SMPTE Epoch is 63072010 seconds before 1972-01-01T00:00:00Z 
>> (UTC).
>> ---------------
>>
>> 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. The intentional point is implementers should treat 
"The SMPTE Epoch" as "63072010 seconds before 1972-01-01T00:00:00Z 
(UTC)." Unfortunately its a non-normative clause, and that's the best we 
could do at the time. My point in general is that its better to discard, 
or ignore, the historical "rubber band era" values.
-Brooks

>> 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