Re: [LEAPSECS] building consensus

From: John Cowan <cowan_at_ccil.org>
Date: Thu, 1 Jun 2006 08:09:22 -0400

Zefram scripsit:

> Readings of UT1 et al are most naturally represented as a real count of
> rotations since some epoch (i.e., as some form of Julian Date).

Such a claim cannot be evaluated without reference to a purpose.

> Because TT, TAI, et al are measures of time unrelated to planetary
> rotation, it is misleading to apply to them the day-based notations
> (such as the sexegesimal time-of-day notation) that are customarily used
> with UT1 et al.

Almost anything *may* be misleading.

> Readings of linear time scales (TT, TAI, et al) are most naturally
> represented as a real count of SI seconds since some epoch.

Such a claim cannot be evaluated without reference to a purpose.

> Post-1972 UTC, counting TAI seconds while maintaining a "day" cycle that
> closely matches the phase of UT1, is directly analogous to calendars
> that count days while maintaining a "year" cycle that closely matches
> the phase of the tropical year.

If this means that leap seconds and leap days are analogous, then I
suppose so. If it means something else, I don't understand it.

> Readings of UTC cannot be directly represented by a single linear count.

Of course they can be. The question of what you can do with that
linear count is another matter.

> Unix time, as standardised by POSIX and as commonly implemented, is a
> faulty encoding of UTC. The fault is that Unix time readings repeat,
> and so are ambiguous, near positive leap seconds.

"Faulty" is a word implying, well, fault. The no-fault view is that it's
an *ambiguous* encoding. Whether that is faulty cannot be evaluated
without reference to a purpose.

> Some applications assume that Unix time is monotonically nondecreasing,
> or that timestamps are unambiguous, and so are poorly served by the
> encoding of UTC in Unix time.

Unix time (better: Posix time) *is* monotonically nondecreasing,
provided you set it with NTP and not by brute force. The point about
ambiguous timestamps is correct.

> Some applications assume that Unix time is a linear scale suitable for
> interval calculations, and so are poorly served by the encoding of any
> form of UT in Unix time.

You should add "to a precision of 1s". Applications whose precision
requirements are, say, 1 day, don't have a serious problem.

> New applications need a more sophisticated understanding of time than
> is currently standard practice.

Some do, some don't, some couldn't care less.

--
But that, he realized, was a foolish            John Cowan
thought; as no one knew better than he          cowan_at_ccil.org
that the Wall had no other side.                http://www.ccil.org/~cowan
        --Arthur C. Clarke, "The Wall of Darkness"
Received on Thu Jun 01 2006 - 05:10:28 PDT

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