# Re: [LEAPSECS] The real problem with leap seconds

From: Michael Deckers <Michael.Deckers_at_FUJITSU-SIEMENS.COM>
Date: Fri, 13 Jan 2006 14:20:21 +0000

On 2006-01-13, Ed Davies wrote:

> This conversation is making something of a meal of a simple
> point. You can treat UTC as a real in either of two ways:
>
> If you don't count the leap seconds then the good news is that
> days are all 86 400 seconds long but the bad news is that the
> real is undefined during the leap second and there's a
> discontinuity (or rather, a surprising continuity in that
> at some point it's 23:59:59.999999.... and a whole second and
> a tiny bit later it's 00:00:00.0000....).
>
> If you do count the leap seconds then that real is the same
> as TAI but the days it's divided up into aren't all 86 400
> seconds long.
>
> Sort of like, is it a particle or a wave? :-)

At the risk of being misunderstood as sarcastic: if
users of UTC were really expected to understand such
strange concepts (Schrodinger time) I would plead for the
immediate abolishment of UTC. Why cannot UTC be simply taken as
the reading of a clock that runs at the same rate as TAI and
that is is set back by a second every once in a while?

> The truth is that UTC only really makes sense as a year,
> month, day, hour, minute and second value. Years have 12
> months, months have 28, 29, 30 or 31 days, days have 24
> hours, hours have 60 minutes, minutes have 59, 60 or 61
> seconds.

Then why can the IERS express UTC in the MJD notation?

> The use of the 23:59:60 notation is described in ISO 8601.
> Is it also specified in TF.460? If so, how do they relate
> it to the notion of DTAI?

Yes, it is specified in [ITU-R TF.460-6. Annex 3]:

"The dating of events in the vicinity of a leap-second shall be
effected in the manner indicated in the following Figures:"

Follow some pictures; for a positive leap second, it looks like:

event
| leap second
+-----|---+
| | |
| | | | |
| | | | |
59 60 0 1
|
---30 June, 23h 59m-->|<--1 July, 0h 0m

Designation of the date of the event
|
|
|
V
30 June, 23h 59m 60.6s UTC

They also say:

"C Coordinated universal time (UTC)

UTC is the time-scale maintained by the BIPM, with assistance from
the IERS, which forms the basis of a coordinated dissemination of
standard frequencies and time signals. It corresponds exactly in
rate with TAI but differs from it by an integer number of seconds.
The UTC scale is adjusted by the insertion or deletion of seconds
(positive or negative leapseconds) to ensure approximate agreement
with UT1."

"E DTAI

The value of the difference TAI – UTC, as disseminated with time
signals, shall be denoted DTAI. DTAI = TAI - UTC may be regarded
as a correction to be added to UTC to obtain TAI. The TAI - UTC
values are published in the BIPM Circular T. The IERS should
announce the value of DTAI in integer multiples of one second in
the same announcement as the introduction of a leap-second (see §
D.2)."

HTH

Michael Deckers
Received on Fri Jan 13 2006 - 06:20:54 PST

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