As more and more systems care about time at the second and sub-second level, finding a lasting solution to the leap seconds problem is becoming increasingly urgent.

摘要

Poul-Henning KampThanks to a secretive conspiracy working mostly below the public radar, your time of death may be a minute later than presently expected. But don't expect to live any longer, unless you happen to be responsible for time synchronization in a large network of computers, in which case this coup will lower your stress level a bit every other year or so. We're talking about the abolishment of leap seconds, a crude hack added 40 years ago, to paper over the fact that planets make lousy clocks compared with quantum mechanical phenomena.History of timekeeping for dummies Timekeeping used to be astronomers' work, and the trouble it caused was very academic. To the rural population, sunrise, midday, and sunset were plenty precise for all relevant purposes. Timekeeping became a problem for non-astronomers only when ships started to navigate where they could not see land. Finding your latitude is easy: measure the height of the midday sun over the horizon, look at the table in your almanac, done. Finding your longitude is possible only if you know the time of day precisely, and the sun will not tell you that unless you know your longitude. If you know your longitude, however, the sun will tell you the time very precisely. Using that time, you can make tables of other nonsolar astronomical events—for example, the transits of the moons of Jupiter, which can then be used to estimate time from that longitude. This is why Greenwich Observatory in the UK and the U.S. Naval Observatory were funded by their respective admiralties. The British empire staked some money on this question, and while the astronomers won on dirty play, the audience vastly preferred John Harrison's chronometers because you did not need to see the transits of the moons of Jupiter to know what time it was. Harrison's chronometer just told you, any time you wanted to know.~(4) Ever since, astronomers have lost ground as "time lords." Time zones, made necessary by transcontinental railroads, reduced the number of necessary observatories to nearly nothing. Previously, every respectable city, with or without a university, had somebody whose job it was to figure out proper time. With time zones and a telegraph, you could service all of the United States from the Naval Observatory. The next loss was the length of a second, which astronomers had defined as "1/31,556,925.9747 of the tropical year in 1900," neither a very practical nor a very reproducible definition. Louis Essen's atomic clock won that battle, and SI (International System of Units) seconds became 9,192,631,770 periods of hyperfine radiation from a cesium-133 atom. A new time scale was created to count these seconds. Civil time was still kept using a different and varying length of a second, depending on what astronomers had measured the earth's rotation to for each year. Having variable-length seconds did not work for anybody, not even the astronomers, so in 1970 it was decided to use SI seconds and do full-second step adjustments—leap seconds—starting January 1, 1972.~(2) In practice, this works by astronomers sending the rest of the world a telegram twice a year to tell us how long the last minute of June and December will be: 59, 60, or 61 seconds. There is a certain irony in the fact 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-plated 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, so your system might do one of the following: 23:59:57 ? ? ? 23:59:57 ? ? ?