Leap-second scare stories

From: Markus Kuhn <Markus.Kuhn_at_cl.cam.ac.uk>
Date: Sat, 30 Jul 2005 11:59:33 +0100

Steve Allen wrote on 2005-07-29 21:37 UTC:
> http://online.wsj.com/article_email/0,,SB112258962467199210-H9je4Nilal4o52nbYCIbq6Em4,00.html

The article repeats an old urban legend:

> In 1997, the Russian global positioning system, known as
> Glonass, was broken for 20 hours after a transmission to the country's
> satellites to add a leap second went awry.

This contradicts statements found in the GLONASS operational bulletin
quoted on


The second scare story is:

> And in 2003, a leap-second
> bug made GPS receivers from Motorola Inc. briefly show customers the
> time as half past 62 o'clock.

It conveniently omits the minor detail that this long preannounced
Motorola software bug actually manifested itself on 27 November 2003
and was not in any way caused by an added leap second, but by an
unwise design choice in the GPS data format and a resulting counter

So I wonder, how much factual substance there really is behind the

> On Jan. 1, 1996, the addition of a leap second made computers at
> Associated Press Radio crash and start broadcasting the wrong taped
> programs.

It seems to go back to a very anecdotal second-hand remark by Ivars
Peterson in


which got quoted by Peter Neumann in

  ACM SIGSOFT Software Engineering Notes, March 1996, p.16

and was later only slightly elaborated by Peterson in


where he admits that he "never could find out precisely why the
problem had occurred and who was responsible for it".

I'm sorry, but I find these three badly documented second or
third-hand rumours of leap-second scare stories neither very scary nor
very convincing.

Perhaps people should try to invent UTC leap-hour scare stories for a
change. They should be at least 3600x more disruptive!

       Stardate 2651-12-31T24:08:16Z, Captain's log. About eight
       minutes ago, we experienced a sudden and entirely unexpected
       catastrophic failure in all our computers that forced us to
       abandon ship. We had just returned from a 6-year deep space
       assignment and entered a geostationary orbit over the Atlantic
       (39 degrees west), when all of a sudden the ship's primary and
       all backup clock networks failed, just as we reconnected to the
       Internet. A warp-core breach is now immanent and my science
       officer predicts that the resulting overwhelming
       electromagnetic pulse will instantly destroy all computers
       located on planet Earth between longitudes 126 degrees west and
       48 degrees East; most of the Western hemisphere.

> Ending leap seconds would make the sun start rising later and later by
> the clock -- a few seconds later each decade. To compensate, the U.S.
> has proposed adding in a "leap hour" every 500 to 600 years, which
> also accounts for the fact that the Earth's rotation is expected to
> slow down even further. That would be no more disruptive than the
> annual switch to daylight-saving time, said Ronald Beard of the Naval
> Research Laboratory, who chairs the ITU's special committee on leap
> seconds and favors their abolishment. "It's not like someone's going
> to be going to school at four in the afternoon or something," he said.

It introduces leap hours into a time scale (UTC) that is so widely
used in computer networks exactly *because* (unlike civilian local
time) it is free of any disruptive DST leap hours!

Let's not forget that this proposal is all about replacing a
reasonably frequent minor distruption (UTC leap seconds) with a very
rare catastrophically big one (UTC leap hours).


Markus Kuhn, Computer Laboratory, University of Cambridge
http://www.cl.cam.ac.uk/~mgk25/ || CB3 0FD, Great Britain
Received on Sat Jul 30 2005 - 03:59:53 PDT

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