Every second counts. Or does it? The answer holds leap second's fate

International timekeepers are meeting in France to debate the future of the bothersome leap second

The International Bureau of Weights and Measures (BIPM – Le Bureau Internationale Poid et Mesure) meets tomorrow (November 18) at its headquarters in Versailles, France, to vote on various things. One of these involves the future of the “leap second”: Will international timekeepers continue to add a second every so often to adjust for natural variations in the Earth’s rotation and revolution?

Many of the world’s largest digital businesses and several government bodies wish to abandon this practice. This decision, one way or another, will actually make a difference to ordinary citizens.

What is a second?

The basic unit of time, the second, was redefined in 1967. Until then, it was defined in terms of the day with 86,400 seconds (24 hours x 60 minutes x 60 seconds) per day. A day, however, is hard to measure precisely. While we understand a day intuitively as one rotation period, the Earth does not rotate at perfectly uniform speed.

The second was redefined in atomic terms in 1967. Using atomic clocks, the second is counted in terms of the frequency of photon absorption in a Caesium isotope (Caesium-133). This can be exactly measured, and it doesn’t vary. The atomic second, which is the international unit of time, is the period taken by Caesium 133 to emit 9,192,631,770 flashes. This can be counted in any lab with appropriate equipment. Of course, the day was redefined as the period when 86,400 new atomic seconds occurred. This is the international definition, used in UTC (Universal Coordinated Time or Temps Universel Coordonné).

Are these measures precise?

The second is very precise. But the day is not, since the Earth has variances in rotation speed, and orbital period. As far as we can figure, the Earth’s rotation is slowing by 1 millisecond or so (1/1000 of a second) every 100 years, but this deceleration is not uniform. So the length of day is increasing. What’s more, there are annual variations in the orbit around the Sun. These are not completely corrected by adding February 29, every leap year.

So what is a “leap second”?

The BIPM adds a “leap second” every so often to adjust for these variations. This involves tedious calculations and a consensus decision taken by scientists interacting at the inter-governmental body. The BIPM adds up errors and corrects them by adding a full second either on June 30, or December 31. Instead of 23:59:59 switching to 0:0:0 as is normal, an extra “23:59:60” is inserted. As many as 27 leap-seconds have been added in this fashion, between 1970 and now.

So what’s the problem?

“Everybody” uses digital time-keeping systems. Your car’s GPS splits time into nanoseconds (1 billionth of a second, or 1 divided by 10 to the power of 9) to tell you where you are. Financial exchanges put time-stamps on orders to one thousandth (millisecond) of a second accuracy, or one-millionth (microsecond) in some cases. Fancy military gear such as target radar requires even greater accuracy (1 picosecond, or 1 divided by 10^12).

Programming a digital system working in nanoseconds, or picoseconds, to add a full second every so often is a painful, error-ridden process. Doing this to millions of different systems is even more messy. If there’s an error, or if one critical system doesn’t synchronise, given our reliance on absolutely precise time-keeping, it could have severe consequences – perhaps as severe as the famous Y2K problem. Glitches are known to cause internet outages, for example.

What problems are known to have been caused by leap seconds?

The leap second change triggered a Reddit crash in 2012, and outages at Mozilla, LinkedIn, Yelp and Amadeus. In 2017, Cloudflare, which hosts many of the world’s biggest websites, was knocked out. Its software thought time had gone backward and couldn’t handle that! This leads to another potential problem – if we ever have to subtract a leap second (which may be necessary), most digital time-keeping systems will probably crash.

What is BIPM going to vote on?

The BIPM is going to consider dropping the practice of adjusting time altogether. Google, Microsoft, Meta and Amazon have lobbied together with the support of many digitally-enabled businesses and platforms to scrap the leap-second.

Their contention is, an inaccuracy in adjustment could cause much more trouble than simply living with inaccuracy. The US National Institute of Standards and Technology (NIST) and French government timekeepers also believe that dropping the leap second would indeed be the safer option. One Meta researcher says that a correction may be necessary, 2,000 years downstream, if the errors are allowed to perpetuate. Other national timekeepers will have to consider this and make a call.

What other solutions could there be?

Meta and Google already use “leap smear” algorithms. Instead of directly adding a full second, this involves small incremental additions of time through a 24-hour period (a 17-hour period for Meta). Each second is lengthened by a tiny amount and if each increment remains within the error-tolerance level of various timekeeping systems, there are fewer chances of a glitch.