On 2014-10-31 11:40 AM, Warner Losh wrote:
On Oct 31, 2014, at 4:17 AM, Martin Burnicki <martin.burni...@meinberg.de>
wrote:
Magnus Danielson wrote:
On 10/31/2014 02:49 AM, Sanjeev Gupta wrote:
Give it a new name, please. Independent of what the "fundamental
unit" is.
TAI and UTC already exists, but the use of TAI has been actively
discouraged.
Huuh?
Just "recently" PTP/IEEE1588 has been specified to use TAI timestamps by
default, and provide a UTC offset as parameter.
As far as I can see it's easy to derive "legacy" UTC from TAI unambiguously if
you know the current offset, and if you have a leap second table available this also
works for timestamps from the past, at least after 1970. So what could be the reason
*not* to use TAI?
BIPM (or their successors, I can never keep all the reorgs straight), the owners of TAI,
have discouraged it. The reasons are that while it is similar to UTC, it differs in some
technical ways. TAI and UTC have a fixed offset relationship, it is true. However, UTC is
computed in real time (with several varieties to choose from if you care about the
nano-seconds), but TAI is a retrospective timescale that's not computed until after the
fact. I get the feeling that the BIPM want TAI to be their baby, free from
"production" concerns that UTC has to deal with
IEEE isn't part of BIPM, so they are free to do what they want, and they make a
contrary recommendation. But if you look closely, they aren't recommending
using TAI, as BIPM defines it, they are using the TAI second labeling for this
real-time realized timescale. So this is a real-time realization of a timescale
whose seconds are numbered like TAI rather than like UTC. It isn't a TAI
timestamp, since technically those have to be compute after the fact from the
raw data rather than done in real time. But it is a timestamp using the TAI
conventions for labeling of seconds. The difference is subtle, and for PTP
makes no difference at all, but does exist.
Yes. Its primary timescale, sometimes called "PTP Time", more properly
the "PTP Timescale", is a "TAI-like" counter (uninterrupted incrementing
count of seconds). Note its origin, or epoch, is 1969-12-31T23:59:50Z,
ten seconds before the POSIX "the Epoch" (if you take that to mean two
years (2 x 365 x 86400) seconds before 1972-01-01T00:00:00Z (UTC), as
NTP does).
-Brooks
The trouble is that those that wants a TAI-like time-scale
sometimes needs to comply to UTC needs, and for a number of reasons they
have difficulty in using it, so they want to make UTC a TAI-timescale.
The naming of a possible future UTC-like time scale without leap seconds is a
different topic, though, and I fully agree with Harlan's and Sanjeev's recent
postings.
Rules change all the time as do the details (UTC pre 1972 is significantly
different than post 1972 for everything except the tracking UT1 attribute),
sometimes the name changes, other times no. Sometimes the change matters to a
lot of people, other times not so many (like the black body correction
introduced in the 1990's). But that's a different set of posts, eh?
Warner
Martin
--
Martin Burnicki
Senior Software Engineer
MEINBERG Funkuhren GmbH & Co. KG
Email: martin.burni...@meinberg.de
Phone: +49 (0)5281 9309-14
Fax: +49 (0)5281 9309-30
Lange Wand 9, 31812 Bad Pyrmont, Germany
Amtsgericht Hannover 17HRA 100322
Geschäftsführer/Managing Directors: Günter Meinberg, Werner Meinberg, Andre
Hartmann, Heiko Gerstung
Web: http://www.meinberg.de
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