On 2014-11-03 03:04 PM, Warner Losh wrote:
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?
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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