Leap seconds and Unix time: what happens and what is planned

UTC occasionally adds a second to stay close to the Earth's rotation. Unix time does not have a place for it.

What a leap second is

Atomic clocks keep very steady time, while the rotation of the Earth varies slightly. UTC is kept within a second of the time based on the Earth's rotation (called UT1) by adding a leap second when the difference is predicted to approach 0.9 seconds. The BIPM's Resolution 4 (2022) states this rule. The extra second is inserted at the end of a UTC day, so that day has 86,401 seconds, and the clock reads 23:59:60.

How many there have been

The IERS table of the difference between international atomic time (TAI) and UTC, which we fetched on 14 November 2026, starts at 10 seconds in 1972 and ends at 37 seconds from 1 January 2017. That means 27 leap seconds have been added since 1972. The last one was at the end of 2016. There have been none since, according to that table.

Unix time and leap seconds

POSIX says that each and every day is accounted for by exactly 86,400 seconds. So Unix time has no way to represent 23:59:60. Systems handle the extra second in different ways: by repeating a second, by stalling the clock, or by "smearing" the extra second over several hours. As a result, around a leap second, different systems may disagree by up to a second, and a few have crashed or looped. The practical consequence is that a timestamp difference between two Unix times is not the exact number of real seconds elapsed if a leap second lies between them.

The plan to stop adding them

In 2022 the General Conference on Weights and Measures decided that the maximum value for the difference between UT1 and UTC will be increased in, or before, 2035. The aim, in the resolution's words, is to ensure the continuity of UTC for at least a century. It asks the CIPM to propose the new maximum value with the other organisations affected. In plain terms: leap seconds are planned to stop being used, with details still to be settled.

What you need to do

• For most programs, nothing. Use Unix time or UTC and let the system keep its clock synchronised.
• For systems that need exact durations over long periods (science, navigation), use a time scale without leap seconds such as TAI or GPS time and convert at the end.
• For servers, use the time service's recommended way of handling leap seconds, and do not mix smeared and unsmeared sources.

Common mistakes

1. Believing that Unix time counts every real second.
2. Using two time servers that handle leap seconds differently.
3. Writing code that cannot cope with a minute of 61 seconds.

This is an independent guide. It is not connected with the IERS, BIPM, IANA, NIST or any company named here. Facts were read on 14 November 2026 from the sources named on each page; calculated values come from Python.