[LEAPSECS] leap second roundup 2017

[Warner Losh]

On Mon, Oct 23, 2017 at 11:37 AM, Brooks Harris <brooks at edlmax.com> 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. 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
>
> 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.


> 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. 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). 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. 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.


> 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.


> 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 may have these properties, but that's 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. In the US, it's a mean solar time (error unspecified) as
determined by some government agency, for example.


> 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...

Warner
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