Will Leap Seconds Be Abolished? - The International Agreement Toward 2035 and Its Technical Impact

What Leap Seconds Are and Why They Were Inserted

Leap seconds are adjustments inserted into UTC to keep it within 0.9 seconds of UT1, the time scale based on Earth's actual rotation. Since UTC was introduced in 1972, 27 leap seconds have been added through the end of 2016. They are inserted as 23:59:60 immediately following 23:59:59 on June 30 or December 31, an unusual second value that does not appear in normal timekeeping.

The need for leap seconds arises because Earth's rotation is not constant. Tidal friction is gradually slowing Earth's spin, lengthening the day by about 2.3 milliseconds per century in the long run. Short-term variations from tectonic motion and atmospheric circulation can speed it up, so the interval between leap seconds is not fixed. From 1972 to 1979 they came annually; since 2017 there has been none, reflecting a temporary acceleration of Earth's rotation.

The 2022 CGPM Decision - International Agreement to End Leap Second Insertion

In November 2022, the 27th General Conference on Weights and Measures (CGPM) adopted a resolution (Resolution 4) to increase the maximum allowed difference between UT1 and UTC in or before 2035. In practice this brings the insertion of leap seconds under the current 0.9-second limit to an end, but as of September 2026 the new limit and its start date have not yet been decided. The decision reflects a judgment that the operational risks leap seconds pose to communications, finance, and navigation now outweigh the value of keeping civil time aligned with Earth's rotation.

The resolution asks for the maximum allowed difference between UT1 and UTC to be increased in or before 2035, keeping UTC continuous for at least a century. How a much larger accumulated gap would eventually be settled is still undecided, and the proposals under discussion would absorb it in one bigger correction rather than many small ones. The concrete figure for the new tolerance is being prepared by the International Committee for Weights and Measures (CIPM) in consultation with the ITU and others, with a resolution expected at the 28th CGPM in 2026.

The IT Headache - Why 23:59:60 Breaks Things

Leap seconds are problematic for software because the value 23:59:60 is not legal in standard time representations. Many software systems assume 60 seconds per minute and cannot represent or process the 61st second. The 2012 leap second exposed a Linux kernel bug that crashed Reddit, Mozilla, Foursquare, and other major services in a coordinated incident.

Air traffic control must carefully manage the difference between GPS time (which does not include leap seconds) and UTC across leap-second boundaries. Financial markets timestamp trades at high precision, and a one-second discontinuity can scramble trade ordering for the duration of the leap second. These risks are why many large operators have lobbied for abolition for years.

Leap Smearing - The Workaround in Use Today

The main workaround for leap seconds is "leap smearing," introduced by Google in 2008. Instead of inserting a single 23:59:60, a smear stretches each second slightly so the adjustment is absorbed gradually. Under the current scheme the window is 24 hours centered on the would-be leap: each second becomes 1/86,400 of a second longer (about 11.6 microseconds), spreading the one-second adjustment over a full day.

Amazon (AWS) applies essentially the same 24-hour smear as Google. Other services do not smear at all and insert the extra second directly, so the approach varies by provider. Comparing timestamps across clouds near a leap second can therefore produce small but real discrepancies. Once leap seconds stop being inserted, smearing itself becomes unnecessary, simplifying interop between cloud platforms.

Life After 2035 - What Will Change

Once leap second insertion ends, the difference between TAI and UTC at that point (37 seconds as of September 2026) is expected to remain in place afterwards. Variations in Earth's rotation will be reflected only in UT1, leaving UTC unaffected. Astronomers and certain navigation systems will continue to use UT1, but everyday IT systems can rely solely on UTC without further adjustment.

Over the long run, the gap between UTC and solar time (UT1) will accumulate. At current rates of Earth's deceleration, the gap will reach a few tens of seconds to a minute in 100 years and roughly one hour in 1,000 years. These shifts unfold over generations, far slower than any human can perceive. The CGPM decision essentially passes the problem of long-term astronomical alignment to future generations to handle, while removing the operational hazard for our era.