Re: [LEAPSECS] Time after Time

From: Markus Kuhn <Markus.Kuhn_at_CL.CAM.AC.UK>
Date: Mon, 24 Jan 2005 09:04:43 +0000

John Cowan wrote on 2005-01-23 18:37 UTC:
> Markus Kuhn scripsit:
> > UTC currently certainly has *no* two 1-h leaps every year.
> There seems to be persistent confusion on what is meant by the term
> "leap hour".


> I understand it as a secular change to the various LCT offsets,
> made either all at once (on 1 Jan 2600, say) or on an ad-lib basis.

No. A "UTC leap hour" is an inserted 60-minute repeat segment in the UTC
time scale, which starts by jumping back on the UTC time scale by one
hour. This has been proposed by BIPM in Torino to be done for the first
time to UTC in about 2600, instead of doing the about 1800 leap seconds
that would be necessary under the current |UTC - UT1| < 900 ms until
then. The proposed "UTC leap hour" simply means that the definition of
UTC is relaxed to (something like) |UTC - UT1| < 59 min, and the size of
the adjustment leap is increased accrodingly from 1 s to 3600 s.

Local civilian times are of no convern to ITU, as they are entirely the
responsibility of numerous national/regional arrangements.

> You seem to be using it in the sense of a 1h secular change to universal
> time (lower-case generic reference is intentional).

I can't understand what could be ambiguous here. A leap hour means to
turn a clock forward or backward by an hour. We have done it twice a
year in many LCTs. The BIPM suggested in Torino that we should do it
every couple of hundred years to UTC as well, which would become
permissible by going from the rule |UTC - UT1| < 900 ms to a relaxed
rule such as |UTC - UT1| < 59 min.

The term "leap hour" does in no way imply what time zone/scale we are
talking about, and in this context we are talking mostly about UTC.

[How a UTC leap hour would affect LCTs is up the maintainers of the
these LCTs. Since the LTCs currently in use have their leap hours on
many different days of the year, a UTC leap hour would mean that at
least some LCTs would have three leap hours in that year. This could
only be avoided if all LCTs would agree to do their DST leaps
simultaneously with the UTC leap.]

In summary: There are basically three proposals on the table:

  a) Keep UTC as it is (|UTC - UT1| < 900 ms) and just make TAI more
     widely available in time signal broadcasts

  b) Move from frequent UTC leap seconds to far less frequent UTC leap
     hours, by relaxing the UTC-UT1 tolerance (e.g., |UTC - UT1| < 59 min)

  c) Remove any future leap from UTC, such that UTC becomes TAI plus a fixed
     constant (i.e., |UTC - UT1| becomes unbounded and will start to grow
     quadratically). In this scenario, LCTs would have to change their
     UTC offset every few hundred years, to avoid day becoming night
     in LCTs.

My views:

  a) is perfectly fine (perhaps not ideal, but certainly workable)

  b) is utterly unrealistic and therefore simply a dishonest proposal
     (UTC is so popular today in computing primarily because it is
     *free* of leap hours)

  c) I could live with that one, but what worries me is that
     it will create a long-term mess in a few millenia, when
     |UTC-LCT| >> 1 day. I am annoyed that this long-term mess and solutions
     around it are not even being discussed. (My hope would have rested
     on resolving the |UTC-LCT| >> 1 day problem by inserting leap
     days into the LCTs every few thousand years as necessary, to keep
     |UTC-LCT| < 36 hours this way, and that these leap days in LCTs could
     perhaps be the same that may be necessary anyway every few
     millenia to fix the remaining Easter drift in the Gregorian
     calendar: )


Markus Kuhn, Computer Lab, Univ of Cambridge, GB | __oo_O..O_oo__
Received on Mon Jan 24 2005 - 01:04:56 PST

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