“programmers should fix their past mistakes for free.” QED: case closed.

Not a lot of common ground there, and not a lot of data supporting either proposition, although Y2K experience, as well as the principles of a capitalist economy, dictate that getting programmers to handle leap seconds correctly will be expensive.

a Possible Compromise?

Warner Losh, a fellow time-and-com-puter nerd, and I both have extensive hands-on experience with leap-second handling in critical systems, and we have tried to suggest a compromise on leap seconds that would vastly reduce the costs and risks involved: schedule the darn things 20 years in advance instead of only six months in advance.

If we know when leap seconds are to occur 20 years in advance, we can code them into tables in our operating systems, and suddenly 99.9% of our computers will do the right thing when leap seconds happen, because they know when they will happen. The remaining 0.1% of the systems, involving ready, cold spares on shelves, autonomous computers on the South Pole, and similar systems, get 20 years to update stored tables rather than six months to do so.

The astronomical flip side of this proposal is that the difference between earth rotation and UTC time would likely exceed the current one-second tolerance limit, at least until geophysicists get a better understanding of the currently not understood fluctuations in earth rotation.

The IT flip side is that we would still have a variable radix time scale: most minutes would be 60 seconds, but a few would be 61 seconds, and code that really cares about time intervals would have to do the right thing instead of just adding 86,400 seconds per day.

So far, nobody has tried, or if they tried, they failed to inject this idea into the official standards process in ITU-R. It is not clear to me that it would even be possible to inject this idea unless a national government, seconded by another, officially raises it at the ITU plenary assembly.

What happens next?
Proposal TF-460-7 to abolish leap sec-
onds will come up for plenary vote at the
ITU-R in January 2012, and if it, modulo
amendments, collects a supermajority
of 70% of the votes, leap seconds would
cease beginning in approximately 2018.

the Philosophical Issues

One of my Jewish friends explained to me that all the rules Jews must follow are not meant to make sense; they are meant to make life so difficult that you never take it for granted. In the same spirit, Van Halen used brown M&Ms to test for lack of attention, and I use leap seconds: if a system has not documented and tested what happens on leap seconds, I don’t trust it to get anything else right, either.

But Linus Torvalds’ observation that “95% of all programmers think they are in the top 5%, and the rest are certain they are above average” should not be taken lightly: very few programmers have any idea what the difference is between “wall-clock time” and “ interval time,” and leap seconds are way past rocket science for them. (For example, Posix defines only a pthread _ cond _ timedwait(), which takes wall-clock time but not an interval-time version of the call.)

When a large fraction of the world economy is run by the creations of lousy programmers, and when embedded systems are increasingly capable of killing people, do we raise the bar and demand that programmers pay attention to pointless details such as leap seconds, or do we remove leap seconds?

As an old-timer in the IT business,
I’m firmly for the first option: we should
always strive to do things better, and do
them right, and pointless details makes
for good checkboxes. As a frequent user
of technological marvels built by the
lowest bidder, however, the second op-
tion is not unattractive—particularly
when the pilots tell us they “have to turn
the entire plane off and on again before
we can start all the motors.”
As a time-nut, a small and crazy fra-
ternity that thinks running an atomic
clock in your basement is a require-
ment for a good life (let me know if
you need a copy of my 400GB record-
ing of the European VLF spectrum
during a leap second…), I would miss
leap seconds. They are quaint and
interesting, and their present rate of
one every couple of years makes for
a wonderful chance to inspire young
nerds with tales of wonders in physics
and geophysics.

Related articles on queue.acm.org

Principles of Robust Timing over the Internet

Julien Ridoux, Darryl Veitch

http://queue.acm.org/detail.cfm?id=1773943

You Don’t Know Jack about network Performance

Kevin Fall, Steve McCanne

http://queue.acm.org/detail.cfm?id=1066069

Fighting Physics: A Tough Battle

Jonathan M. Smith

http://queue.acm.org/detail.cfm?id=1530063

References

1. International earth rotation and reference Systems Service. Information on u TC-TaI; http://data.iers.org/ products/16/14433/orig/bulletinc-041.txt.

2. International earth rotation and reference Systems Service. relationship between TaI and u TC; http:// hpiers.obspm.fr/eop-pc/earthor/utc/TaI-uTC_tab. html.

3. Microsoft. How the Windows Time service treats a leap second (2006). (november 1); http://support. microsoft.com/kb/909614.

4. Sobel, D. Longitude. Walker and Company, 2005.

5. Williams, M. Power glitch hits Toshiba’s flash memory production line. Computer World (Dec. 2010); http:// www.computer world.com/s/article/9200738/Power_ glitch_hits_Toshiba_s_flash_memory_production_line.

 

Poul-henning Kamp ( [e-mail]) has programmed computers for 26 years and is the inspiration behind bikeshed.org. His software has been widely adopted as “under the hood” building blocks in both open source and commercial products. His most recent project is the Varnish HTTP accelerator, which is used to speed up large Web sites such as Facebook.