Why it's time to leave leap seconds in past and what problems they cause computers?

The International Bureau of Weights and Measures in France has decided to abandon the so-called leap seconds – extra seconds, the introduction of which was to reflect changes in the rate of rotation of the Earth. Excessive seconds, according to experts, cause serious failures in computer systems.

What are leap seconds and why are they needed? 

Leap seconds are added to the Coordinated Universal Time (UTC) scale to align it with UT1 mean solar time. This practice has been in place since 1972 and has been applied 27 times since then: an extra second is added according to astronomical observations at the end of the day at Universal Time on June 30 or December 31 so that the difference between UTC and UT1 is no greater than 0.9 seconds. On such days, 23:59:59:59 is followed by 23:59:60, and only then by midnight.

And while people can understand where this second comes from and why it is needed, computers are less clear about it, which actually leads to malfunctions.

The extra seconds cause trouble?

Many experts argue that the extra seconds clearly cause more problems than benefits.

Back in 2012, experts reported numerous problems with Linux, as its kernel was unable to handle the extra seconds. There were also problems with sites such as Foursquare, Reddit (it was inaccessible for almost 40 minutes), LinkedIn and Yelp. Despite all the measures taken, the problems repeated themselves in 2015 and 2016 as well.

In 2017, there were also problems: with another time conversion, some of the resources belonging to Cloudflare customers went offline because the service's software platform compared clock readings from two different sources, saw that time had reversed, and was unable to correctly process this result.

The possibility of doing away with leap seconds has been discussed since at least 2013, and recently these discussions have become even more active. A few months ago, Google, Microsoft, Meta, and Amazon launched a public initiative proposing to do away with leap seconds because of the problems they create.

According to Meta's engineers, they have to smudge the extra seconds in time over 17 hours by slowing or speeding up the clock, but even this practice is fraught with difficulties and problems.

Many experts now recommend abandoning the further use of leap seconds and stopping at the current level (27), which, in their opinion, will suffice for at least the next millennium. Moreover, it is possible that because of the slowing down of the rotation of the Earth the seconds in the future will have to be subtracted rather than added (although no such thing has ever been done in world practice so far).

Final decision concerning abandonment of leap seconds

The 27th General Conference of the Bureau of Weights and Measures has made a decision (PDF) to stop leap seconds. However, this will probably happen no sooner than 2035.

Russia voted against the resolution, because its global navigation satellite system GLONASS includes and takes into account additional seconds, unlike other systems such as GPS.

Instead of extra seconds, the experts propose setting a new maximum value for the difference between UT1 and UTC that will ensure UTC continuity for at least a century. At the 28th General Conference in 2026, the International Bureau of Weights and Measures is expected to vote on a resolution that would set a maximum value for the difference between UT1 and UTC until 2035.