would be expensive to find out, because
relative to Y2K, the questions are a lot
harder than “ 2 digits or 4 digits.”
How do we tell if code that does s +=
3600 intends this to mean “one hour
from now” or “same time, next hour?”
The original programmer did not ex-
pect there to be any difference, so the
documentation will not tell us.
the Cost of uncertainty
The next time Bulletin C tells us to insert
a leap second, probably in 2012, a lot of
people will have to kick into action. Any
critical bits installed since December
2008 and any bits older than that that
failed to “do the right thing” with the
December 2008 leap second will need to
be pondered, and a plan made for what
to do: test, fix, hope, or shut down.
Unsurprisingly, many plants and
systems simply give up trying to predict
what their multivendor heterogeneous
systems will do with a leap second,
and they sidestep the issue by moving
or scheduling planned maintenance
downtime to cover the leap second.
For them, that is the cheapest way to
make sure that no robot arms get out of
sync with the assembly line and that no
space-shuttle computers hiccup while
in space.
I’m told from usually reliable sources that the entire U.S. nuclear deterrent
is in “a special mode” for one hour on
either side of a leap second and that the
cost runs into “two-digit million dollars.”
But What Do Leap seconds
actually Do?
Leap seconds make sure the sun is due
south at noon by adjusting noon to happen when the sun is due south at the reference location. This very important job
is handled by the International Earth
Rotation Service (IERS).
Leap seconds are not a viable long-term solution because the earth’s rotation is not constant: tides and internal
friction cause the planet to lose momentum and slow down the rotation,
leading to a quadratic difference between earth rotation and atomic time.
In the next century we will need a leap
second every year, often twice every year;
and 2,500 years from now we will need a
leap second every month.
On the other hand, if we stop plugging leap seconds into our time scale,
there is no
problem with leap
seconds that we
It professionals
cannot tolerate. We
just have to make
sure all computers
know about leap
seconds and that
all programs,
operating systems,
and applications
know how to deal
with them.
noon on the clock will be midnight in
the sky some 3,000 years from now, unless we fix that by adjusting our time
zones.
Actually, the sun is not due south at
noon, and certainly not with a second’s
precision, for more than an infinitesimal number of people who are probably
totally unaware of it. Our system of one-hour-wide time zones means that only
those who live exactly on a longitude
divisible by 15 have the chance, provided that their governments have not
put them in a different time zone. For
example, all of China is one time zone,
despite the 75–120E span of longitude.
Of the remaining few lucky people,
many are out of luck during the part of
the year when their government has decided to have daylight saving time—
although that could possibly put a select
few of those who lost on the first criterion back in luck during that part of the
year. Finally, it is really only a couple of
times a year that the sun is precisely due
south, for interesting orbital and geophysical reasons.
The people who really do care about
UTC time being synchronized to earth
rotation are those who use UTC time as
an estimator for earth rotation: those
who point things on earth at things in
the sky—in other words, astronomers
and their telescopes, and satellite operators and their antennae. Actually,
that should more accurately be some
of those people: many of them have
long since given up on using UTC as an
earth rotation estimator, because the +/-
1-second tolerance is not sufficient for
their needs. Instead, they pick up Bulletin A or B from the IERS FTP server,
which gives daily values with microsecond precision.
the Cost-Benefit equation
Most of those involved on the “Abolish
Leap Seconds” side of the debate claim
a cost-benefit equation that essentially
says: “cost of fixing all computers to
deal correctly with leap seconds = infinity” over “benefits of leap seconds = next
to nothing.” QED: case closed.
The vocal leaders of the “Preserve
the Leap Seconds” campaign (not to
be confused with the “Campaign for
Real Time”) have a different take on
the equation: “cost of unknown consequences of decoupling civil time from
earth rotation = [a lot...infinity]” over