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