Re: [LEAPSECS] Risks of change to UTC

From: Neal McBurnett <>
Date: Fri, 20 Jan 2006 14:52:40 -0700

On Fri, Jan 20, 2006 at 03:12:15PM -0500, John Cowan wrote:
> Rob Seaman scripsit:
> > Only a minority (small minority, one would think) of
> > systems currently include any DUT1 correction at all - although these
> > will perhaps tend to be the most safety-critical applications. [...]
> >
> > That is, of course, one of the major issues for astronomers - we rely
> > on UTC providing a 0.9s approximation to UT1 and most of our systems
> > don't use DUT1. Even our high precision applications (in either
> > interval or universal time) don't tend to require conversions other
> > than as a preprocessing step. Remediating our systems for such a
> > fundamental change to UTC would involve much larger changes than Y2K
> > did - algorithms and data structures would have to change, not just
> > the width of some string fields and sprinkling some 1900's around.
> I don't understand this. The first class of applications, those
> that actually receive DUT1 from somewhere, probably have a hard-coded
> assumption that |DUT1| never exceeds 0.9s or at worst 1.0s. They would
> need remediation. The second class, which just assumes UTC = UT1
> and doesn't care about subsecond precision, would simply need to be
> front-ended with a routine that got DUT1 from somewhere and mixed it
> with TI to generate their own UT1. This is technically remediation,
> but of a rather black-box kind.

I don't do this professionally, but here is my guess, and an attempt
at an analogy. Hopefully the pros will correct me if I'm wrong.

Imagine if someone changed the definition of the meter so that it
changed over time.... How would you modify your systems to make sure
that all your units were consistent - that some contractor didn't foul
it up, and give you the wrong measurement, subtly or catastrophically
changing the results? Like that fabled Mars mission that failed
because some calculations were done with the wrong units?

Today, for many systems, we can assume that DUT1 obeys the
three-decade-old standard and is less than 0.9 s. If that changes
because the definition of UTC changes, we have effectively a whole new
unit, a non-UT civil time measure. And made it more confusing by
reusing the old name and invalidating megatons of documentation!

These astronomical and navigation systems currently get inputs of time
from all sorts of other systems and people: other instruments, other
computers, sub-contractors, etc. Changing the meaning of UTC would
lead to a cascade of systems that changed their own timescales and
I/O formats, meaning that auditing the flow of time data between them
would get more and more complex for a long time.

To sum it up, PLEASE don't fundamentally change the DEFINITION of UTC,
or you risk whole new kinds of confusion. Hopefully by now the folks
on this list that don't like leap seconds at least have agreed that
any change should be to a new time scale like TI, and announced
decades in advance.

Neal McBurnett
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Received on Fri Jan 20 2006 - 13:53:09 PST

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