Re: [LEAPSECS] Mechanism to provide tai-utc.dat locally

From: Rob Seaman <seaman_at_noao.edu>
Date: Wed, 27 Dec 2006 10:14:47 -0700

Clive D.W. Feather writes:

>> WG14 is willing in principle to make changes to time_t, up to and
>> including
>> completely replacing it by something else. *BUT* it needs a
>> complete and
>> consistent proposal and, preferably, experience with it.

This is at the heart of my distaste for the so-called leap hour
proposal. There is no coherent proposal, no implementation plan, no
discussion of adverse effects, no budget, no collection of pertinent
use cases, no exploration of requirements - no technical design
discussion at all.

Meanwhile, the astronomical community (like other prudent
communities, presumably) has instituted long range planning efforts
like the U.S. Decadal Surveys. Major (and minor) telescope and
instrumentation projects involve multi-year design commitments with
significant budgets of their own simply to develop coherent and
complete proposals to submit to the funding agencies. The U.S.
national centers have recently been subject to an NSF Senior Review
process that is likely to have a major affect on all our operating
budgets to free up funding for new initiatives. No pain, no gain.

On the other hand, we have a "proposal" to change the fundamental
underpinnings of world-wide civil timekeeping. A (publicly
unavailable) proposal that can't even be bothered to suggest how DUT1
will be conveyed in a future in which this quantity would assume
vastly greater importance. It's an embarrassment.

>> Any proposal has got to deal with a whole load of issues, many of
>> which
>> haven't been properly documented. For example, it should be
>> possible to add
>> and subtract times and intervals (e.g. "what time is 14 months and
>> 87 days
>> from now?").

Poul-Henning Kamp wrote:

> ... which is exactly the kind of thing you can not do with any
> {origin+offset} format, due to leap seconds.

Leap seconds are an effect, not a cause. The intrinsic difficulty is
with mapping interval time to Earth orientation (mean solar time).
Whatever mechanism is used to synchronize civil time to the sun -
including embargoing leap seconds as leap hours - there will always
be this complication.

Consider an ISO 8601 compliant date/time representation, e.g.:

        % date -u +"%FT%TZ"
        2006-12-27T16:47:27Z

The first part is the (Gregorian) calendar date - the second clearly
represents a fraction of a day. From my point of view, this is the
beginning and end of the argument establishing an identity between
civil time and mean solar time. Others are willing to permit a slow
secular drift - in the calendar, too, of course, not just in the
clock. Mucking with leap seconds is equivalent to redefining the
concept of a "day".

The point is that over a long enough period, a broad enough temporal
horizon, we all agree that civil time must be synchronized to solar
time. The emergence of the absurd leap hour proposal from among
folks who loathe leaps of any sort demonstrates that. They weren't
eager to center their notional position around leap hours - rather,
they felt obligated. In short, there is no escaping the need to
grapple with the fundamental distinction between Earth orientation
and "atomic" interval counts. Just as, as one enlarges ones spatial
horizon one cannot fail to run into relativistic effects.

Timekeeping is a subtle business. Others on this list surely
understand that better than I.

Rob Seaman
NOAO
Received on Wed Dec 27 2006 - 09:15:07 PST