# Re: [LEAPSECS] A lurker surfaces

From: Ed Davies <ls_at_edavies.nildram.co.uk>
Date: Sun, 31 Dec 2006 12:50:09 +0000

Poul-Henning Kamp wrote:
> In message <4CF2642E-FBA9-4E52-A013-A65B0E2439B7_at_noao.edu>, Rob Seaman writes:
>> Jim Palfreyman wrote:
>
>> Just a reminder that UTC has no - none - nada - discontinuities.
>> Various computer mis-implementations may, but the standard is very
>> carefully constructed to avoid spring-forward or fall-back gaps or do-
>> overs.
>
> Rob, If you feel uncomfortable with calling leapseconds
> discontinuities, then we can use the term arrhythmia instead.

If we assume that every month has 30 days and obtain a day
number by multiplying the month number by 30 and adding
the day in month (call this the SDN - Silly Day Number) and
then look at SDN-MJD (modified Julian day number) we would
see discontinuities.

The only way to see discontinuities in UTC-TAI is by making
an equally silly assumption in numbering the seconds of
UTC: assuming all UTC minutes are 60 seconds or, equivalently,
all UTC days are 86 400 seconds.

The unfortunate thing is that people actually do think of it
this way. E.g.:

http://hpiers.obspm.fr/eop-pc/earthor/utc/TAI-UTC_tab.html

The whole idea of the expression UTC-TAI being meaningful
and evaluating to a number of seconds is a convenient but
rather sloppy shorthand. Any strongly typed programming
language ought to give a type error on that expression.

UTC times of day are variable radix - in just the same way
as days and months are in the Gregorian calendar.

Except, of course, that the Gregorian calendar is fixed and
completely predicable. I have an awful lot of sympathy for
the idea of making leap seconds predictable over longer
periods, even if it risks UTC-UT1 becoming larger than at
present allowed.

Ed.
Received on Sun Dec 31 2006 - 04:51:36 PST

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