Global time will be set slightly forward by a "leap second" this summer in a small step for clocks that may cause a giant headache for the part of mankind that manages big computer systems.
The extra second will be tacked onto the final minute of June 30 to keep Co-ordinated Universal Time (UTC), which is measured by extremely accurate atomic clocks, in sync with astronomical time (UT1), which is based on the - somewhat variable - rotation of Earth on its axis.
Without the periodic addition of leap seconds, the difference between UTC and UT1 would continuously widen.
Scientists attribute this to the ongoing deceleration of the Earth's rotation, caused by natural phenomena such as tidal friction.
"At the moment the difference is 0.5 seconds," said Andreas Bauch, a physicist at the Braunschweig-based PTB (Physikalisch-Technical Bundesanstalt), Germany's national metrology institute and responsible for weights and measures.
"In several weeks it will be 0.6 seconds."
By international agreement, the difference between UTC and UT1 must never exceed 0.9 seconds.
Leap seconds were introduced in 1972. There have been 25 of them so far. They are added on the last day of June or December and announced a half-year in advance by astronomers at the International Earth Rotation and Reference Systems Service (IERS), who measure Earth's rotation and compare it with the time kept by atomic clocks in more than 70 national metrology institutes.
Atomic time defines a second as 9,192,631,770 oscillations of a cesium-133 atom, astronomical time as 1/86,400 of a mean solar day.
"If the difference weren't corrected, at some point the sun would rise in the afternoon," said Wolfgang Dick from the IERS Central Bureau in Frankfurt.
The correction can cause trouble, however. While most clocks make the leap without stumbling, some computer software doesn't. The leap second in 2012 paralyzed a number of websites and temporarily knocked out the booking system of the Australian airline Qantas.
"It's amazing what a little second can do," Bauch remarked, pointing out that the more processes depend on integrated times, the greater the chance of a missed or faulty correction at a critical juncture.
Adverse chain reactions are conceivable in electricity grids for instance, he said, because calculations of current flow are made with a temporal resolution - or precision - of microseconds, or millionths of a second. The same goes for calculations of grid load.
A leap second causing an incorrect calculation that then signals an error could prompt a power-line shutdown - "an automatic measure meant to protect the high-voltage network."
A temporal resolution of milliseconds, or thousandths of a second, is used in air-traffic control, money-market transactions and satellite navigation.
"At such enormous orbital speeds, a discrepancy of a second would put (satellites') positions somewhere else altogether," Bauch said.
Safety-related systems could also be affected, noted Fiete Wulff, spokesman for Germany's Network Agency, which regulates and oversees fair competition in the country's electricity, telecoms and other markets.
"It has to be checked which systems are susceptible," he said.
For private companies, leap seconds bring the extra costs of having to alter clocks by hand. It's hardly surprising that leap seconds are not universally beloved, and abandoning them was formally proposed for the first time in 2001 by the United States.
"It's not normal for a specific issue like this to be on the agenda for 14 years, but the parties were, and are, at loggerheads," Bauch said. Leap seconds' opponents, led by the United States and France, plan to renew their efforts to scrap the practice at this year's World Radiocommunication Conference of the International Telecommunication Union in November in Geneva. Germany, too, now favours abandonment, Wulff said, "provided it doesn't cause any technical problems."
Among the countries that want to keep leap seconds are Britain, Russia, China and Canada.
The main argument for abandonment is that it would eliminate the risk of errors as well as the extra work and expense, Dick said, adding, "It's clear that a correction must be made at some point, though. Just an hour's difference in a person's circadian rhythm is distinctly felt."
One possibility, he said, would be to add a "leap minute" every 100 years, or a "leap hour" after longer intervals. "But after such a long time, no one would know any more where there could be a problem and a need to adjust systems." One reason Britain opposes dropping leap seconds is that it would further devalue the Prime Meridian (0 degrees longitude) at the Royal Observatory in Greenwich, England, home of Greenwich Mean Time (GMT). Now a time zone, GMT, the mean solar time on the Prime Meridian, lost its status as a time standard when governments adopted UTC.
Britain's opposition rests on the argument that for the first time in human history, our concept of time would be severed from the rising and setting of the sun.
At the moment, the "pro" and "contra" camps are about the same size. Unanimity is required to change the universal time system, however.
"Getting a unanimous decision won't be possible in November," Dick said.
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