[LEAPSECS] the big artillery

[Brooks Harris]

On 2014-11-03 03:04 PM, Warner Losh wrote:
> On Nov 3, 2014, at 12:53 PM, Brooks Harris <brooks at edlmax.com> wrote:
>
>> On 2014-11-03 02:19 PM, Warner Losh wrote:
>>> On Nov 3, 2014, at 11:11 AM, Brooks Harris <brooks at edlmax.com>
>>>   wrote:
>>>
>>>> CAUTION about the PTP Epoch. Its not "just nitpicking".
>>>>
>>> ...
>>>
>>>> We've been advised by PTP experts that A) yes, its confusing, and B) most implementations use a integral-second interpretation, as in Table B.1. I understand the "escape clause" they use to justify this is the "(POSIX) algorithms" phrase in Note 1 of 7.2.2 Epoch. By "(POSIX) algorithms" they mean "gmtime()" and (strict) POSIX "ticks" at 1Hz, so, integral seconds. In any event its really the only interpretation that yields a manageable, practical, implementation that is consistent with TAI and UTC, NTP, and common-use of POSIX.
>>>>
>>> A few years ago, I had to produce TAI-like data from a measurement system. We defined the value as "seconds since 1970" but the technical definition was "number of SI seconds since 1 Jan 1972 00:00:00 UTC + 10 + #seconds-in-1970&71" to avoid the ambiguity. Given that our chief time scientist suggested this, and they were quite involved in PTP...
>> I assume you mean "number of SI seconds since 1 Jan 1972 00:00:00 UTC + 10 - #seconds-in-1970&71" ? And the "#seconds-in-1970&71" is (2 * 365 * 86400), right? That would be coincident with the PTP Epoch as interpreted above, that is, "seconds since 1970 (TAI)".
> TAI is ahead of UTC, so you have to add in the leap seconds to UTC to get TAI. At 1 jan 1972 00:00:00 UTC this offset was 10s, exactly. To bias the date back to 1970, you have to add in 2 * 365 * 86400, to give a value of 63072010 for 1 jan 1972 00:00:00 UTC. I don't think you want to subtract it, since that leads to an epoch of 31 DEC 1973 00:00:00 due to the leap day in 1972, right?

OH, well, which way round are we calculating? 1972-01-01T00:00:00Z (UTC) 
+ 10 = 1972-01-01 00:00:00 (TAI) - (2 * 365 * 86400) = 1970-01-01 
00:00:00 (TAI) = 1969-12-31T23:59:50Z (proleptic UTC). RIght?

(Of course its just these knids of miscommunications that lead to the 
great confusions and debate over UTC and "civil time"...)

-Brooks

>
> Warner
>
>> -Brooks
>>
>>> Warner
>>>
>>>
>>>
>>>
>>>
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