that the UTC (Universal Time Coordinated) time scale depends on the rotation of one particular rock in the less
fashionable western part of the galaxy.
I am pretty sure that, should humans
ever colonize other rocks, leap seconds
will not be in the luggage.
how Leap seconds
Became a Problem
Until the advent of big synchronized
networks of computers, leap seconds
bothered nobody. Many computers
used the frequency of the electrical grid
to count time, and most had their time
initially set from somebody’s wristwatch. The number of people who actually cared probably numbered fewer
than two dozen worldwide.
Therefore, Unix didn’t bother with
leap seconds. In the time _ t definition from Unix, all minutes have 60
seconds, all hours 3,600 seconds, and
all days 86,400 seconds. This definition carried over to Posix and The Open
Group where it is presumably gold-plat-ed for all eternity.
Then something shifted deep under
the surface of the earth. We can only
guess what it might have been, but there
was no need for leap seconds for seven
straight years: from the end of 1998 to
the end of 2005. This was, more or less,
the time when the Internet happened
and everybody bought PCs with Windows. Most of the people who hacked
Perl to implement the dot-com revolution had never heard of leap seconds.
This is what Microsoft had to say on
the subject of leap seconds: “[...]after
the leap second occurs, the NTP (
Network Time Protocol) client that is running Windows Time service is one second faster than the actual time.” 3
Unix systems running NTP will paper over the leap second, but there is no
standard that says how this should be
done. Your system might do one of the
scenarios shown in the accompany fig-
sensitivities in leap seconds.
ure. Or it might do something entirely
different. Some systems have resorted
to slowing down the clock by 1/3600th
for the last hour before the leap second,
hoping that nobody notices that seconds suddenly are 277 microseconds
That’s in theory. In practice it depends on the systems getting notice
of the leap second and handling it as
intended. In this context systems are
also the NTP servers from which the
rest of the computers get their time: at
the 2008 leap second, more than one in
seven in the public NTP pool servers got
the effort to “fix” Leap seconds
By early 2005 when the first leap second in seven years finally began to look
likely, some people started to worry
about a “Y2K-lite” event. Some bright
person inside the U.S. military-indus-trial complex thought, “Wait a minute,
why do we need leap seconds in the first
place?” and proposed to the ITU-R (
International Telecommunication Union,
Radiocommunication Sector) that they
be abolished, preferably before December 2005.
Nice try, but one should never underestimate the paper tiger in a UN organization.
The December 2005 leap second
came, Armageddon did not, but it was
painfully obvious to everybody who
paid attention that there were massive
amounts of software that needed fixing,
before leap seconds would not cause
trouble. Even the HBG time signal from
the Swiss time reference system did it
Another leap second occurred in December 2008, and the situation had not
changed in any measurable way, but at
least the Swiss got it right this time.
Since then the proposal, known to insiders as TF.460-7, has been the subject
of “further study” in “Study Group 7A,”
(halt for 1 sec)
and all sorts of secret scientific brotherhoods, from AAU to CCTF, have had
their chance to weigh in. Many have, but
few have clear-cut positions.
What is the Problem
with Leap seconds?
The problem is that more systems care
about time at the second level.
Air Traffic Control systems perform
anti-collision tests many times a second
because a plane moves 300 meters in a
second. A one-second hiccup in input
data from the radar is not trivial in a
tightly packed airspace around a major
Medical products and semiconductors are produced in time-critical processes in complex continuous production facilities. On December 8, 2010,
a 70-msec power glitch hit a Toshiba
flash chip manufacturing facility, and
20% of the products scheduled to ship
in January and February 2011 had to
be scrapped: “Once the line is stopped,
we can’t just resume production,” said
Toshiba spokesman Hiroko Yamazaki. 5
Technically, there is no problem with
leap seconds that we IT professionals
cannot tolerate. We just have to make
sure that all computers know about
leap seconds and that all programs, operating systems, and applications know
how to deal with them.
The first part of that problem is we
have only six months to tell all computers and software about leap seconds,
because that is all the warning we get
from the astronomers. In practice, we
often have 10 months’ notice; for example, we were told on February 2 that
there will be no leap second in December of this year. 1
Unfortunately, this advantage is negated by some time signals—for example, the DCF77 signal from Germany,
announcing the leap second only one
hour ahead of time.
The other part of the problem—
changing time _ t to know about leap
seconds—has nasty results: time is
suddenly not a fixed radix quantity anymore. How much code finds the current day by d = t/86400 or tests if two
events are further apart than a minute
byif (t1 >= t2 + 60)? Nobody
knows. How much of such code needs
to be fixed if we change the time _ t
definition? Nobody knows.
The Y2K experience indicates it