On 2017-10-23 02:07 PM, Warner Losh wrote:
On Mon, Oct 23, 2017 at 11:37 AM, Brooks Harris <[email protected]
<mailto:[email protected]>> wrote:
On 2017-10-23 09:58 AM, Rob Seaman wrote:
Multiple timescales exist now for multiple purposes. Multiple
timescales
will exist under all scenarios. Debasing Universal Time is not a
solution to any "real world" problem. If you want a new timescale,
define a NEW timescale.
Indeed.
We don't need a new timescale. We have plenty.
We love timescales! Make more! :-)
UTC has always been the best one to have. It's usefulness isn't
because astronomers think it's great (it is inaccurate, after all, as
a Universal Time in the astronomical sense, but it is useful enough to
astronomers as an approximation that certain optimizations can be made
were it not quite so accurate).
To me, the frustrating thing about the discussion at ITU and
elsewhere is the apparent outright refusal to consider a "second
timescale". It is considered and then dismissed out of hand in:
Document 7A/39-E
United States of America
DRAFT NEW REPORT ITU-R TF.[UTC]
The International Time Scale, Coordinated Universal Time (UTC)
https://www.itu.int/md/meetingdoc.asp?lang=en&parent=R15-WP7A-C&source=United%20States%20of%20America
<https://www.itu.int/md/meetingdoc.asp?lang=en&parent=R15-WP7A-C&source=United%20States%20of%20America>
In the last paragraph before the Conclusion they say
"... Another alternative proposed to ensure backward compatibility
with the current UTC time-scale is to use another international
coordinated continuous time-scale on an equal basis. This was
suggested as a suitable method to provide a choice of time scales
that could be applied for a particular system. The implementation
of such an option has not been determined as either possible or
practical, and the possibility of confusing two international
standard time scales makes such a solution unlikely."
I think that is actually quite wise. What time is it really? would
replace the "did we get the leapsecond" when different systems try to
exchange data.
"implementation of such an option has not been determined as either
possible or practical" and that will remain true useless somebody tries.
The irreconcilable difficulty arises from UTC being a modification
of the Gregorian calendar algorithm. The world (mostly) uses
Gregorian, but then along comes this unpredictable and irregular
Leap Second to upset the apple cart. No clever algorithm can fit
that 86401th second label (23:59:60) back into the Gregorian
86400-second-day. The Leap Second must go, and so it does, either
by ignoring it or smearing it, thereby creating many incompatible
and inaccurate timescales in the real world.
Gregorian, strictly speaking, doesn't have leap seconds.
Right. 1/86400ths of an observed day. Leap Seconds throw a wrench in its
works.
It's just a new numbering of days to keep the year alignment in sync.
It cares not how the days are divided.
There are two underlying physical phenomenon; time by atomic
science, and time by astronomical observation. The counting
mechanisms between the two are incommensurate because humans (and
astronomers) expect the time-of-day to indicate the position of
the Sun in the sky. This is not just a matter technical
considerations but a matter of *principle*.
True. Does time measure elapsed time, or the angle of the spinning
rock. I posit that the latter is not interesting so long as the former
gives an answer that's close enough (eg within an hour).
There's that value judgment again. "civil time is good enough within an
hour". The point I'm trying to make is that civil time is not a matter
of accuracy but of principle, belief, confidence, consistency with local
culture and expectations - people care about local time, not UTC, and
they take it, or would like to take it, as being perfect. They *want* it
to reflect the solar time of day.
On the other hand, in the media business I come from its not so much a
matter choice or opinion as a need to conform to local time expectations
while maintaining highly precise timing. In that application, civil time
must be accurate to at least nanoseconds to resolve media units and
maintain constant frequency while also satisfying the local time
expectations.
Even with leap seconds, we are only forestalling the time when we must
add them so frequently that we cannot keep up.
Earlier in the same document they say:
".. Maintaining a conceptual relation with the Earth's Rotation
Angle (represented as UT1) does not appear to be a necessity for
the sake of civil time."
Isn't that a *value judgement*? It seems its this sort of value
judgment that upsets many who feel that solar time is important.
At the Science of Time symposium and elsewhere we've heard many
impassioned presentations about how important solar time is to
humans; practically, culturally, and religiously.
I happen to think 'uniformity of timescale' is more important than
synchronization to a wobbly rock. But they are right: It doesn't
matter what time the clocks say it is, so long as everybody's clock
matches. We already accept a variance from the local solar time for
time zones, and things like DST suggest that we as a civil society
have a tolerance of an hour or two between observed local time and
clock on the wall time. In this sense, civil time doesn't need to be
tied, exactly to the second, to what the earth is doing. Society will
function correctly if the location of the arbitrary synchronization to
a time drifts east a little bit. We'll be good for thousands of years
by that standard. It may be desirable for other reasons, but it isn't
required for a civil time.
Civilians *want* time to reflect astronomical time in a Gregorian
YMDhms form. UTC with Leap Seconds has served that purpose
admirably for decades, tying the worlds timekeeping systems
together, albeit imperfectly. The one second accuracy compromise
of UTC has long since been accepted as a practical matter, and the
system has been in effect since 1972. Proposals to change it meet
with impassioned resistance not so much on technical grounds but
on cultural preference. "Civil time" is *supposed* to be mean
solar time, the way its been for centuries, the way UTC has been
since 1972, and the way the Gregorian calendar prescribes it.
Civil time hasn't been a strict solar time for 150 years.
Never has been really, but it was the objective for centuries. Local
time is obviously a gross approximation, but a very useful one. Before
atomic time, navigation time (the almanacs) and "civil time" were
largely one and the same. Leap Seconds decoupled them.
Not since the railroads standardized the time to be the time at an
arbitrary location near where you are. So be careful in claiming the
full history of time supporting Solar time. We've not been on a
strictly solar time where I am right now system for a century and a
half. We transitioned to a 'what time is it given my locations is
rounded to the nearest(ish) 15 degrees.
I think atomic time dissemination by UTC with Leap Seconds is
unlikely to change because its so widely deployed, accepted since
1972, works great for many applications, and efforts to change it
have failed since at least 2000. But still, somehow the Leap
Seconds must be eliminated to reestablish compatibility with the
unmodified Gregorian calendar.
I tend to agree with this. However, UTC has changed several times
since it's conception, and could easily change again with a tweak to
the spec (though some applications might be affected). Leap Seconds
aren't fundamental to nature, but a means to an end (finding the
location the earth is pointing because that correlates in some way to
human activity). It could change in the future to a timetable
projected into the future, for example, of leap seconds. Do one every
18 months for the next 10 years. 5 years in, we'll publish the next
decade's worth of data. With a change to the spec to relax DUT1, this
could easily be *a* evolution of UTC. It's still a mean solar time
that's non-divergent, but one whose instantaneous difference from UT1
wanders in a larger range. This would have the benefit of
predictability (which starts to make leap seconds testable), while
allowing things to not wander off, just wander a bit more than they
have been. Leap Seconds replaced a time and frequency jump schedule
that relaxed the delta from UTC to UTC from < ~100ms to < 1s. How far
out could we plan leaps seconds if we relaxed that further to 10s?
I find it a bit incongruous that while the discussion seems to
insist there be only one "international timescale", in fact there
are already two (or three, if you count TAI separate from UTC, but
UTC is the disseminated form of TAI). ITU Rec 460 defines DUT1
(1/10th second resolution UTC-UT1), the IERS maintains and
announces it (Bulletin D), and the radio signals broadcast it.
This could provide the raw material on which to define a timescale
that is more accurate than, and also traceable to, UTC.
UT1 isn't a timescale in the strictest sense. TAI and UTC are the same
timescale, but with different second labeling. Apart from small
differences in realtime realization, they are always lock step. UT1
is an artificial construct of averaging local time as well, with the
seasonal variations subtracted out.
As per Richard Langley's note, "UT1 gives us the true orientation of the
Earth in space as measured (now) directly by space geodetic systems."
We have the "smeared timescales" (Google, AWS, Bloomberg, etc).
Each generally varies the frequency in the 12 or 24 hours
surrounding the Leap Second to "hide" it from the receiving
systems. This eliminates the Leap Second from view, reestablishing
the Gregorian calendar, and downstream systems and applications
behave more reliably. However, these "smears" do not match each
other so tractability amongst them and to UTC is compromised, and
the frequency shifts are more extreme than might be necessary.
The existence and proliferation of the smears suggest, I would say,
that leap seconds are not a good fit to a large segment of time users
and suggests that leap seconds are a poor way to maintain synchronization.
I agree. Leap Seconds have to go, and the 1/86400th of a day of the
Gregorian calendar must be restored. This is the only feasible way for
computer systems, applications, and traditional timekeeping systems to
operate. However, the UTC standard with Leap Seconds cannot be
significantly changed, as per above. So, I suggest, use DUT1 to define
an accurate interoperable timescale that is traceable to UTC and better
reflects the underlying phenomenon..
Use of DUT1 could improve this situation. DUT1 values are
announced by IERS, become effective on a specific date, and
typically span several weeks or months periods. If the DUT1 values
were used to specify a (very slight) frequency shift of the
dissemination clock during those intervals the resulting
time-points would essentially "slowly smear away" the Leap Second
during the entire period between announced Leap Seconds.
DUT1 varies a lot from day to day. It rarely spans even days when you
look at it at enough resolution. It changes by hundreds of
microseconds a day typically. The 100ms version that used to be
included in terrestrial broadcasts
Still is, I'm pretty sure.
may have these properties, but that's a crude approximation.
Ah, no, I don't think that's right, as per Richard's comment above. Look
carefully at Bulletin D and IERS Conventions. DUT1 stays within 1/10th
second of UTC. Its not a crude approximation.
Current proposals seek to eliminate the Leap Second, decoupling
timekeeping from solar time, or defer the Leap Second, increasing
its inaccuracy. Rather than reducing the accuracies, this DUT1
driven timescale idea instead *increases* the accuracies by using
higher resolution than one second, essentially "mini-leaps" by
frequency shift. My back-of-the-envelope calculations suggest the
precision with respect to UTC would be in the microseconds,
satisfying most definitions of "legal time" tolerances.
Most of those claimed 'legal time' tolerances haven't really been
really well litigated.
Right, not well litigated and not very clearly specified either.
Financial regulations are helping drive definitions of "legal time". In
the EU its defined in MiFID II as 100 microsecond. Everyone is have
trouble achieving and verifying that.
MiFID II: 10 Things You Need to Know About Time Synchronization
http://tabbforum.com/opinions/mifid-ii-10-things-you-need-to-know-about-time-synchronization
In the US, it's a mean solar time (error unspecified) as determined by
some government agency, for example.
For USA finance its currently NIST UTC time +- 50 milliseconds. See
FINRA 6800. CONSOLIDATED AUDIT TRAIL COMPLIANCE RULE
http://finra.complinet.com/en/display/display_viewall.html?rbid=2403&record_id=17601&element_id=12826&highlight=time
I think the idea that the "possibility of confusing two
international standard time scales" is not so important. As it is
there are many timescales in use and it is likely they are already
confused. A new internationally sanctioned timescale, in addition
to the existing UTC with Leap Seconds, would make the physical
realities of atomic time and astronomical time explicit and
standardized. I think having the selection between two accurate
international timescales would be far better than a single choice
that cannot possibly work. I think DUT1 could provide the raw
material for such a timescale and the IERS already has the
information and procedures in place to accomplish it.
While an interesting idea, I think it would prove unmanageable in
practice. But then again, we've disagreed on this point for a long time...
My opinions have changed over time as I've studied this more carefully.
I used to think UTC and Leap Seconds were cool because they neatly
defined the atomic v.s. astronomical time, and they do. But now I think
the approach was somewhat ill advised because it fundamentally altered
the Gregorian calendar and that was bound to have ramifications, and it
does.
However, as above, I don't believe UTC can be substantially changed
because of reverse compatibility, continuity, and installed base. So,
Leap Seconds are here to stay, and that's good for dissemination of
atomic time, but they must also go away if computer systems,
applications, and traditional timekeeping systems are to operate without
complication.
That's where the obvious dawned on me - create a timescale more like
what GMT once was, or was intended to be, before atomic time was
invented. Forward into the past! DUT1 gives you the raw information to
create a timescale that pretty closely tracks the spinning rock while
remaining consistent with UTC and Leap Seconds. Thus, two timescales to
more accurately represent the two physical phenomenon, atomic time and
wobbly rock time.
Note the idea is this could be implemented at the primary time servers
where the expertise resides to accomplish it, avoiding the infeasible
challenge of implementing some algorithm in every operating system and
application. It would be more accurate, rather than less accurate, as
other proposals are. I think expectation is toward better accuracy and
that its well within modern technology to accomplish it. We just have to
agree how to count.
-Brooks
Warner
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