Is Time Running Out for the Leap Second?

The fate of the leap second is more than just the fate of the leap second. At stake is Coordinated Universal Time, the international standard for timekeeping, which the continued existence of the leap second is slowly undermining.

Coordinated Universal Time, or U.T.C., is tenderly constructed from readings made by atomic clocks kept at national laboratories around the world. These clocks tick off, or “realize,” their best seconds and send the measurements to the B.I.P.M. There, timekeepers painstakingly assemble the readings — averaging, weighting, adjusting for discrepancies — into an ideal second for everyone everywhere to agree on and employ, occasionally adding leap seconds as needed. This assembly process takes time. And so once a month the Bureau publishes the perfect time in the form of a newsletter, called Circular T, that tells each national clock how much it diverges from the international standard, to help it improve its aim the following month.

Coordinated Universal Time is the world’s official time scale, and will continue to be whether or not it incorporates leap seconds. Global time zones are described in reference to it. (New York time currently is U.T.C. minus five hours.) And the beating heart, the second, is the most important in the constellation of standard measurements overseen by the B.I.P.M., alongside the meter (length), kilogram (weight), kelvin (temperature), candela (intensity of light), ampere (electric current) and mole (amount of substance).

The idea, formalized a century and a half ago by national signatories to an international treaty called the Meter Convention, is that each unit of measurement should be identical everywhere in the world; one meter in Spain is precisely one meter in Singapore. The seven standard units are integral to fair commerce, reproducible science and reliable technology. The second is extra-special because it underpins all the other units except the mole. For instance, the meter is defined as the distance light travels in a vacuum during one-299,792,458ths of a second, and the kilogram was recently redefined in terms of the second.

In addition, the second is tethered to a time scale, or flow of seconds. A key tenet of modern life is that not only must the unit of time be identical no matter where it is measured, so must the flow of seconds of which the one is a part.

But the leap second is putting that tenet is at risk. The kludge is so technically difficult for digital technology to incorporate that other, ersatz methods of timekeeping — unofficial, but free of leap seconds and easier to implement — have begun to displace U.T.C., according to a recent article in the journal Metrologia. To supporters of Resolution D, removing the leap second from U.T.C. would make the standard time scale friendlier to modern digital technology, at least in the century following 2035. Coordinated Universal Time would still be universal, just not coordinated with Earth time.

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