UTC questionnaire

Markus Kuhn Markus.Kuhn at cl.cam.ac.uk
Sun Dec 5 10:51:08 UTC 1999

"D. J. Bernstein" wrote on 1999-12-05 01:43 UTC:
> The bottom line is that I'm running several UNIX systems that provide
> accurate civil-time displays and accurate real-time differences; I only
> had to fix a small amount of code.

I do understand the advantages in your approach of using a true
TAI-locked second counter as the Unix time_t value. The really only
thing that makes me somewhat uncomfortable about this approach is that
if you exchange time_t values across systems or if you archive them, the
interpretation of these values becomes fully dependent on the reliable
availability of an up-to-date leap-second table everywhere, and that
table currently has to be updated every year or so. I simply see
logistic problems and hassles to guarantee that an enormous variety of
systems gets updated every 12-18 months with a new table and that for
times more than 6 months into the future, the TAI time_t <-> broken-down
representation is not even predictable at the moment accurately.

Your proposal of running Unix clocks internally on TAI becomes in my
opinion MUCH more feasable and safer, if leap second tables hadn't to be
updated that frequently. Hence my suggestion to USNO/IERS to relax the
|UTC-UT1| boundaries and to announce UTC leap seconds a few decades in
advance. A freshly installed computer should not require a leap-second
table update for the next 30 years and the earth spins reliably enough
to make this feasible if we do not insist on the |UTC-UT1| < 0.9 s


Markus G. Kuhn, Computer Laboratory, University of Cambridge, UK
Email: mkuhn at acm.org,  WWW: <http://www.cl.cam.ac.uk/~mgk25/>

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