On Nov 3, 2014, at 12:53 PM, Brooks Harris <[email protected]> wrote:
> On 2014-11-03 02:19 PM, Warner Losh wrote: >> On Nov 3, 2014, at 11:11 AM, Brooks Harris <[email protected]> >> 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 >> >> >> >> >> >> _______________________________________________ >> LEAPSECS mailing list >> >> [email protected] >> https://pairlist6.pair.net/mailman/listinfo/leapsecs > > _______________________________________________ > LEAPSECS mailing list > [email protected] > https://pairlist6.pair.net/mailman/listinfo/leapsecs
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