[LEAPSECS] the big artillery

[Warner Losh]

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?

Warner

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