[LEAPSECS] A new use for Pre-1972 UTC

Joseph M Gwinn gwinn at raytheon.com
Tue Feb 17 16:54:12 EST 2009

leapsecs-bounces at leapsecond.com wrote on 02/17/2009 04:17:20 PM:

> On Tue 2009-02-17T20:53:43 +0000, Poul-Henning Kamp hath writ:

> > This is a variant of the UUID madness that somebody came up with

> > because they didn't want to run a registry or use the existing

> > well-structured process (ISO OID's) and though that the eventual

> > collisions "probably doesn't matter much".


> And the upshot is software that believes that the system clock is

> always right. Or, more weakly, saying the system clock must be

> monotonic -- but that is basically saying that if the clock ever gets

> fast then it must stay fast. So if the clock gets wrong it must stay

> wrong, or else at least it must get right in a fashion that is

> consistent with that software's notion -- despite any side effects

> that might have on the requirements of other systems that depend on

> time.

No, monotonic does not imply that. One can speed up and slow down, so
long as one does so gently enough. This is exactly what NTP does in
normal operation.

> The fallacy that "my sense of time is always right" is what led to a

> different kind of collision, the grounding of ships off Scilly in

> 1707, and the development of marine chronometers. The navigators who

> used marine chonometers knew perfectly well that those chronometers

> did not keep the "right" time as measured by clocks on land being

> reset by telescopes. Instead they knew that if their chronmeters were

> treated well they kept uniform time, and those navigators knew that

> getting the "right" time meant keeping a log of the difference between

> the "right" time of the clocks on land and their chronometer.

They used the best cronometers then available. Harrison's first attempt
at a chronometer was in 1730, and success came many years later, in 1760
or so.



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