The leap second is abolished: from 20 May 2027 clocks may drift – what it means for GPS, power grids and you

• The decision: Resolution 5 of the 28th CGPM – continuous UTC from 20 May 2027.
• New tolerance: the gap between atomic time and Earth-rotation time (|UT1-UTC|) may grow to 3,600 seconds, or one hour.
• History: 27 leap seconds since 1972, every one of them positive. The last came at the turn of 2016–17.
• The risk avoided: a 30 per cent chance of a never-tested negative leap second by 2035.
• Norway: legal time is fixed at UTC+1, and the Norwegian Metrology Service’s atomic clocks feed into UTC.

What exactly was decided, and when does it take effect?

The CGPM is the supreme body of the International Bureau of Weights and Measures (BIPM), with one vote per member state. According to BIPM Resolution 5 (October 2026), the conference “decides that continuous UTC will become effective on 20 May 2027”. The maximum permitted difference between UTC and UT1, the time scale that tracks Earth’s rotation, is set at 3,600 seconds, “ensuring the long term continuity for UTC for several centuries”.

The decision brings forward a 2022 plan to widen the tolerance “in, or before, 2035”. The resolution says the ITU’s telecoms sector, the IEEE community behind the Precision Time Protocol and industry formally asked for it to happen sooner.

What is a leap second, and what does continuous UTC mean?

UTC (Coordinated Universal Time) is the time scale the BIPM computes from atomic clocks around the world, and it underpins civil time in most countries. A leap second is an extra second – 23:59:60 UTC – inserted to keep atomic time in step with Earth’s rotation.

The IERS Bulletin C of July 2026 confirms there will be no leap second at the end of December 2026. The next possible date would have been 30 June 2027, after the new rule takes effect. So the leap second on New Year’s Eve 2016 was, in all likelihood, the last one ever.

Continuous UTC is UTC without further leap seconds: atomic time ticks on evenly, at a fixed offset from International Atomic Time (TAI), drifting slowly away from Earth’s rotation.

Why is the leap second being abolished now?

Mainly because Earth has started spinning faster. According to UC San Diego (March 2024), a day measured by Earth’s rotation was 2.5 milliseconds longer than an atomic day in 1972, but only 80 microseconds longer in 2023. The geophysicist Duncan Agnew estimated in Nature that a negative leap second might be needed “some time in 2028”.

A negative leap second would give one minute only 59 seconds. It has never been done. The BIPM cites a March 2025 workshop that put the probability of one at 30 per cent by 2035, and warns of a “high risk” of disruption to critical infrastructure that is “largely unprepared”. Getting ready, the resolution says, would cost about as much as preparing for the millennium bug.

Even positive leap seconds cause trouble

At the last leap second, a number that should never have gone below zero turned negative inside Cloudflare’s DNS software. According to Cloudflare (1 January 2017), about 0.2 per cent of DNS queries were affected at the peak. To avoid such jumps, big providers “smear” the extra second over a longer period – each in its own way, which the BIPM says leaves different systems out of step with one another.

Who decides what time it is in Norway?

Norwegian time is set by statute. Section 6 of the Act on units of measurement, measurement and standard time states that standard time in Norway is one hour ahead of UTC. Because Norwegian time is tied directly to UTC, Norway’s clocks will follow continuous UTC without any change in the law, on our reading of the text.

The practical timekeeper is Justervesenet, the Norwegian Metrology Service. According to Justervesenet, its laboratory runs five atomic clocks – two hydrogen masers and three caesium clocks – and sends timing data to the BIPM in Paris every five days. Its national realisation, UTC(JV), is often only a few nanoseconds off UTC, and it offers UTC over NTP at ntp.justervesenet.no.

The same kind of precision metrology makes it possible to detect gravitational waves from colliding black holes.

What does it mean for GPS, power grids and you?

For most people, nothing noticeable. Phones and computers take their time from the network, and the drift from the sun will amount to mere seconds in our lifetimes. The CGPM itself notes that time zones and daylight saving already create gaps of more than an hour between solar time and clock time – as Norway will notice on Sunday morning, when the clocks go back.

• GPS: runs on its own time scale with no leap seconds. GPS time was set equal to UTC in 1980, according to the GNSS researcher Richard Langley, and is now 18 seconds ahead (our calculation from the IERS table). That gap will now be frozen.
• Power and telecoms: the BIPM justifies the decision by saying transport, energy distribution, telecommunications and space projects need UTC “without any future adjustment”.
• Data centres: no more smearing schemes or special handling for second 60.

What happens next?

The BIPM will work with the ITU, the International Astronomical Union, the IERS and others, and keep monitoring UT1-UTC. How the gap will be handled if it ever nears an hour has not been decided – a “leap hour” has not been adopted. Radio equipment that broadcasts UT1-UTC and cannot be replaced sooner has a transition period until 2035, and no later than 2040.

The same meeting also adopted a resolution on an international lunar reference time scale traceable to UTC – timely as humans head around the Moon again.

Frequently asked questions

Is the leap second being abolished, and from when?

Yes. Under BIPM Resolution 5 of October 2026, UTC becomes continuous on 20 May 2027, with no further leap seconds.

Why is the leap second being abolished in 2027?

Critical infrastructure needs time without jumps, and a faster-spinning Earth raised the risk of a never-tested negative leap second. Timekeepers therefore brought the change forward from 2035.

What is continuous UTC?

Continuous UTC is atomic time that ticks evenly without leap seconds. It may drift up to one hour from Earth’s rotation.

Who sets the time in Norway?

Norwegian law fixes standard time at UTC+1. The BIPM computes UTC, and Justervesenet maintains Norway’s reference time, UTC(JV).

When was the last leap second?

The last one was added at the turn of 2016–17. In all likelihood it will remain the very last.