July 1 will be the longest day in the entire year - by just a single second - but the clock is ticking for "leap seconds" as calls for their abolishment grow due to concerns that the adjustment could potentially scramble computer systems.
The world clock, or Coordinated Universal Time (UTC), is precisely calculated based on a range of parameters including Earth's rotational speed. Adjustments are sometimes needed to keep the clock in sync with the length of the day, which has subtle fluctuations that scientists believe stem from geological factors and meteorological conditions.
Atomic clocks (see below) capable of accurately measuring a second were developed in 1958, leading to the use of a new time standard based on the devices, called the International Atomic Time (TAI). UTC has followed TAI ever since, but Earth's rotational speed is constantly in flux. This means the two time standards eventually fall out of sync.
"To fix that time lag, leap seconds were introduced in 1972," National Institute of Information and Communications Technology (NICT) Director Yuko Hanado said.
Leap seconds ensure that the time difference between Earth's rotational speed, measured by TAI, and UTC stays below 0.9 seconds. The twice-yearly synchronization window is precisely plus or minus one second at midnight UTC (9:00 a.m. JST) on Jan. 1 or July 1.
If a second is being added, the second after 8:59:59 a.m. would be counted as 8:59:60 a.m. and the following second would be 9:00 a.m. The 59th second would be erased if a decision is made to subtract instead, though all 25 previous adjustments were additions.
Since 1972, the difference between the Earth's rotation and the atomic clock time is projected to stand at 26 seconds.
Leap seconds were long thought to be a necessary measure, but NICT research manager Tsukasa Iwama pointed out that "leap seconds are starting to become a problem now that computers are so widely used."
The insertion of a leap second is decided half a year in advance by an international body, but the adjustment runs the risk of throwing off computer clocks which could in turn affect business operations. For example, an additional second in 2012 caused an Australian airline's computer system clock to malfunction.
Some businesses temporarily close before and after an adjustment is made to ensure smooth operations. Since the leap second this year falls on a weekday for the first time in 18 years, the government is urging companies to make adequate preparations.
To leap or not to leap
Calls have been growing since 1999 for leap seconds to be abolished. "It would take tens of thousands of years for the time difference to become large enough for night to become day and vice versa, so there isn't a large obstacle for abolishment," Iwama said. The issue will be put to a vote at an international conference in November.
Nations including Japan, the United States, France and China are in favor of abolishing leap seconds, while Britain is against the change because to do so would spell the demise of Greenwich Mean Time and subsequently the country's timekeeping role. Russia is also opposed, since the country's satellite control systems aren't equipped to handle TAI. A compromise proposal has been raised to continue using TAI together with UTC.
The fate of leap seconds could hinge on how the world handles the additional second on July 1. If a decision is made to abolish leap seconds, then clocks showing 59 minutes and 60 seconds would no longer be needed sometime after 2022.
■ Atomic clocks
Device that measures time based on the transition frequencies of electromagnetic waves absorbed and released by atoms and molecules. A second is defined as the duration of 9,192,631,770 cycles of radiation between the nucleus of a cesium atom and the surrounding electrons. The weighted average of over 400 clocks around the world form International Atomic Time, used to calculate UTC.
■ Optical lattice clocks
Researchers are developing technology that can measure a second with more precision than atomic clocks. One of them is the optical lattice clock, developed by University of Tokyo Prof. Hidetoshi Katori and his team in 2001.
Optical lattice clocks measure seconds based on the oscillation frequencies of light absorbed by strontium atoms. Atomic clocks lose one second every 30 million years, whereas a pair of optical lattice clocks can be synchronized side by side "with such precision that they wouldn't deviate even a single second over the span of the birth of the universe until the present, or 13.8 billion years," Katori said.
A subsidiary body of the International Committee for Weights and Measures (BIPM), which oversees the definition of a second, is evaluating optical lattice clocks among other devices as next-generation technology that can measure seconds.
Tetsuya Ido, a research manager at NICT, said he "hopes Japanese timekeeping technology will be adopted and used for the international time standard."