Re: [LEAPSECS] interoperability

From: Rob Seaman <seaman_at_NOAO.EDU>
Date: Sun, 8 Jan 2006 15:19:36 -0700

On Jan 8, 2006, at 12:48 PM, Poul-Henning Kamp wrote:

> What you overlook here is that computers tend to trancend
> governmental boundaries.

This only strengthens my arguments. Anything that ties the world
together more tightly functions to create a single world "stage two"

> Sensibly designed operating systems keep time in the form of the
> first stage clock,

Perhaps. We have no examples of this. Stage one would be TAI. As
we have just been reminded, TAI is "not ready for prime time".

> Badly designed operating systems keep time in local time which
> makes interchange of information a nightmare across timezones.

You are arguing apples and oranges. These operating systems, in
effect, use "stage three" clocks. Current (well designed) operating
systems start with stage two (UTC). The question of delivering wall
clock time is a trivial elaboration on first delivering common
international business time. (I'm trying on different terminology
than "civil time" until I hit one that sticks.)

> That's actually the good thing about the constant offset, it should
> make it much easier to see if timestamps mix things that shouldn't be.

How precisely? I am willing to be convinced but I simply don't see
what you mean. The fundamental error was in allowing sexigesimal
representation to be used for conflicting purposes. Two planes
following parallel tracks in opposite directions better be using the
same clock.

> Sure, and you can timestamp then on either timescale, because there
> is a 1 to 1 translation between the two timescales [1].

Perhaps I miss your meaning here, too. The event of migrating a time
zone is a discontinuity just as with a leap second or leap hour.

> Denmark spans only a few hundred kilometers from east to west (not
> counting Greenland this time), yet sunrise and sunset varies about
> 30 minutes from one side to the other.

This is true. It is irrelevant to the underlying international
clock. These are simply constant (if position dependent) offsets.
Big wup. I think this issue is confusing the discussion.
Longitudinal offsets with a time zone are irrelevant; the equation of
time is irrelevant; daylight saving time is irrelevant. They may all
be interesting effects, but they don't affect the underlying
worldwide clock tick in the same fashion as cumulative secular
offsets caused by rotational rate changes.

What matters is not when sunrise occurs, but rather that every day
has one (and only one).

> Conversion from stage two to stage one (and back) is perfect,

Don't believe a detailed enough proposal is on the table to either
define the meaning of "perfect" in this context, or determine if the
notion being discussed meets the requirements for being so regarded.

> If Denmark or Elbonia decides to use a timezone which is offset
> from stage one by 1h3m21s, then it still works,

Again, what is "it", precisely?

> (but people travelling abroad will probably vote differently in the
> next election)

Exactly. The pressures to maintain a common international vision of
time will trump local variations. It is the resulting common
international time clock that you won't let me refer to as "civil
time". All requirements placed on UTC flow backwards from here. You
can't just edit UTC (or GMT) out of the debate.

        stage one is atomic time (e.g., TAI)
        stage two is international civil time (e.g., UTC)
        stage three is local legal time (e.g., Mountain Standard Time)

One could even regard stage four as daylight saving, but I think this
is equivalent to a separate realization of stage three time at a
particular location.

I suspect you will reject this out of hand, but perhaps you might
first ask the nice folks at the Copenhagen observatory for their

> In a couple of hundred years, the Danish Parliament (or its
> successor in interest) will simply decide "from YYYY-MM-DD HH:00,
> the Danish Civil time will use offsets -3h and -2h (instead of
> presently -1h/-2h) and the transition will happen on the switch
> from summertime to wintertime by _not_ adjusting the clock".

The only way this differs from the leap hour proposal is that you are
assuming that different localities can (or would) carry these
adjustments out separately.

Let's see - how does this work? The Earth is slowing (has slowed),
so TAI is progressively "lapping" UTC. (TAI was 32s ahead of UTC,
now it is 33s ahead.) One might entertain embargoing leap seconds
(or the equivalent time zone operations) until an hour's worth
accumulates. (A better choice would be to seesaw a half hour either
way around the "natural center", but the same logic applies.)

So imagine an hour's worth of "time pressure" has accumulated after a
few hundred years. Under the current standard, 3600 small steps
would have bled away the pressure. Under the ITU notion, a leap hour
would be needed. A leap hour means moving UTC backward one hour (to
let TAI pull ahead). As I've said before, under the daylight saving
analogy this is only naively a "fall back" event, it would be better
to explicitly add a 25th hour. But let's continue through to the
logical conclusion of implementing this via "fall back" events (or
the equivalent time zone shifting).

A fall back event means that the clock (local, standard,
international, whatever clock you want) first traverses an hour - and
then traverses it again. Under the current three stage system it is
only the most local stage three clocks that are affected. You are,
in effect, promoting this discontinuity to stage two - to the
worldwide business timescale. More to the point, you have said that
stage one can be mapped back-and-forth to stage two. But we've just
shown that this is no longer a one-to-one mapping since the hour is
traversed twice, corresponding to two hours of TAI duration.

Ah! But you've suggested that the other half of the annual daylight
saving pendulum be used. This doesn't work because we're on the
wrong side of the pendulum's arc. The point being that you don't
need to *not* adjust the clock in the Autumn - you need to not adjust
the clock in the Spring. It is the springtime "gap" in the mapping
(also not a very desirable feature for a time scale) that is omitted
during one of these events - not the harvest-time doubly traversed hour.

(We'll omit discussion of the fact that not all localities observe
daylight saving time in the first place.)

This is the same point I was trying to make about the 25 hour day.
No historian or lawyer is going to look favorably on a situation that
results in ambiguous timestamps. Perhaps, you say, such timestamps
should all be kept in TAI. But in that case, we are back to the
original question of why a stage two clock is needed at all. By
asserting stage two is needed, all the rest logically follows.

>> nothing short of redefining the second can avoid the quadratic
>> acceleration between the stage one and stage two clocks.
> Does not follow. It's only a matter of how often timezones change.
> A far better way would be to redefine the length of the day to a
> more suitable value a couple of thousand years from now.

Um. How does one redefine the length of the day without changing the
length of the second? Answer: by changing the number of seconds in
the day. I won't belabor the difficulty of selling the idea of
having different hours of different lengths. Perhaps our big-domed
descendants will have finally outgrown our Babylonian sexigesimal

...but isn't what you are asserting precisely equivalent to saying
that you are willing to support the issuance of one or more leap
seconds - per day? If this will be "more suitable" a couple of
thousand years from now - why not now?

> In the case of moving timezones, this effect wouldn't make it self
> felt until 1200 years from now (the second "leap hour"), and we are
> in no position to prescribe how people a millenia from now will
> measure time.

Prescribe? No. Predict? Yes. This is a requirement of considering
changes to any standard - to predict future usage versus current usage.

> If we preposterously try, they will at best get a good laugh about
> their quaint forefathers preposterousness.

The issues facing our great^N grandchildren are ones whose
implications we can, and should, seek to understand. Our own
forefathers were living, breathing people as fully alive as we - and
as fully alive as our technologically hobbled descendants will be.
"Quaint" has no place in it. Julius Caesar wasn't quaint to lay down
his calendar. Our responsibility to future generations is clear. It
is attempting to duck that responsibility that is preposterous.

Rob Seaman
National Optical Astronomy Observatory
Received on Sun Jan 08 2006 - 14:19:57 PST

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