World abandons leap second due to changes in Earth's rotation speed: clocks will follow new rules
The Earth does not rotate at a constant speed, and this has forced scientists to change the rules of world time. Starting May 20, 2027, individual seconds will no longer be added or subtracted from it. It would seem, what difference can one second make? However, for computer networks, banking systems, and satellite navigation, such an adjustment can create serious problems.
The decision was made on October 15 at the General Conference on Weights and Measures. According to Resolution No. 5 of the International Bureau of Weights and Measures (BIPM), the world time standard UTC will now be allowed to gradually diverge from the time determined by the Earth's rotation, up to one hour.
But does this mean that one day people will have to adjust their clocks by 60 minutes at once? And why does the rotation of our planet affect clock readings at all?
Why the Earth doesn't always fit into 24 hours
We are used to thinking that a day consists of exactly 24 hours, or 86,400 seconds. However, the duration of the Earth's actual rotation on its axis varies slightly.
The rotation speed is influenced by the tidal forces of the Moon and the Sun, processes inside the planet, and movements of the oceans and atmosphere.
In the long term, the Earth gradually slows its rotation. But in certain periods it can rotate faster, and this has been observed in recent years.
Modern atomic clocks work differently: they count seconds with extremely high precision and do not depend on the planet's rotation speed.
Therefore, the two ways of determining time—astronomical and atomic—gradually diverge.
To keep them in agreement, in 1972 a special adjustment called the "leap second" or "extra second" was introduced.
When the drift became too large, specialists added one more second to world time.
At such moments, the last minute of the day could look unusual: after 23:59:59, the value 23:59:60 would appear, and only then would midnight arrive.
Since 1972, leap seconds have been added 27 times. The last such adjustment occurred on December 31, 2016.
The Earth began to rotate faster—and a new problem arose
Previously, leap seconds were only added, compensating mostly for the slowing of the Earth's rotation.
But in recent years, scientists have detected the opposite trend: in some periods the planet completes a rotation on its axis faster than expected.
This raised the possibility of a completely new operation—for the first time in history, a second would have to be subtracted rather than added.
With a negative leap second, clocks would have to skip one of the time values. For a person, such a difference is almost unnoticeable, but for digital infrastructure this is a potentially dangerous situation.
According to experts' estimate cited in the BIPM resolution, the probability of needing a negative second by 2035 is about 30%.
Such an adjustment has never been performed before. Many computer systems are not designed to handle time suddenly jumping over a second.
Why one second can disrupt computers
Precise time is needed not only for displaying the clock on a smartphone.
It is used to synchronize servers, mobile networks, power grids, financial transactions, and satellite systems.
For example, computer programs record sequences of events by timestamps. If different devices process the addition or removal of a second differently, their clocks can temporarily diverge.
This can lead to errors in data transfer, disruption of record order, and failures of individual services.
As the National Physical Laboratory of the United Kingdom explains, the leap second already poses technical difficulties for systems requiring continuous time synchronization.
Some technology companies bypass the problem by smearing the leap second over a longer interval. But different ways of handling it create new complexities when computer systems interact.
That is why international experts decided to switch to a continuous UTC scale.
The allowed difference between clocks and Earth's rotation has been increased to one hour
Before the new decision, the drift between Coordinated Universal Time (UTC) and astronomical time (UT1) had to remain within about 0.9 seconds.
Now this permissible limit has been increased to 3600 seconds—a whole hour.
However, this does not mean that on May 20, 2027, clocks will be set forward or backward by an hour.
The international decision merely allows atomic time to gradually diverge from time based on Earth's rotation. The need to constantly eliminate small deviations using leap seconds will disappear.
According to experts, this approach will ensure the continuity of UTC for several centuries.
If in the distant future the drift really approaches the set limit, international organizations will have to determine further actions.
Astronomical time will still be monitored. It remains necessary, for example, in astronomy and some navigational calculations.
Will this cancel daylight saving time in Ukraine?
No. The world time reform has nothing to do with the usual switch between summer and winter time.
The leap second was needed to align atomic time with Earth's rotation. Seasonal clock changes are a completely different procedure, regulated by each country independently.
Therefore, the international decision does not cancel Ukraine's transition to winter time on October 25, 2026.
In the night from Saturday to Sunday at 04:00 Kyiv time, clock hands will be moved back one hour—to 03:00. This procedure is still provided for by Cabinet of Ministers Resolution No. 509.
The new UTC rules will not require Ukrainians to additionally reset phones, computers, or wristwatches in May 2027.
What will change for ordinary people
For most people, the transition will go unnoticed. Days will not become longer or shorter by a whole hour, and the usual way of determining local time will remain.
The main changes will occur in precise synchronization systems that underpin the operation of the internet, communications, energy, transport, and other technological services.
Abandoning leap seconds should reduce the risk of errors caused by sudden time adjustments and simplify coordination of computer systems worldwide.
Thus, for the first time in more than half a century, the international community is changing its approach to aligning world clocks with Earth's rotation. Instead of adding or subtracting seconds to track small changes in the planet's speed, scientists decided to allow the two time systems to gradually drift apart.
And although one second seems negligible to a person, for modern digital infrastructure, giving up such adjustments became an important step toward more predictable operation.
Based on materials from: International Bureau of Weights and Measures – Resolution No. 5 of October 15, 2026, BIPM – decisions of the General Conference on Weights and Measures, National Physical Laboratory of the United Kingdom, Timeanddate.com – explanation of the world time reform.