Leap Second
A leap second is a one-second adjustment to Coordinated Universal Time, used to keep civil time close to Earth's uneven rotation.
What a leap second is
A leap second is an extra second, or in principle a removed second, applied to Coordinated Universal Time. It exists because atomic clocks tick with extreme regularity, while Earth's rotation is slightly irregular. UTC uses leap seconds to keep everyday clock time close to Earth's angle of rotation.
Why UTC needs adjustment
UTC is based on International Atomic Time, but civil time also has a historical connection to the solar day. The length of Earth's day changes by tiny amounts because of tides, movements inside Earth, atmosphere and ocean effects, and other geophysical processes. Those small differences accumulate until UTC and UT1 need to be realigned.
UT1, UTC, and TAI
TAI is a continuous atomic time scale. UTC follows the same rate as TAI but differs from it by a whole number of seconds. UT1 tracks Earth's actual rotation. Leap seconds are the bridge that has kept UTC within about 0.9 second of UT1.
How a leap second appears
A positive leap second is normally inserted at the end of a UTC month, most often June 30 or December 31. The clock can show 23:59:60 before rolling to 00:00:00. That timestamp is unusual, and many computer systems need special handling because ordinary clocks and databases often assume every minute has exactly 60 seconds.
Who announces them
The International Earth Rotation and Reference Systems Service monitors Earth orientation and announces leap-second decisions. National timing laboratories, time-signal services, operating systems, and network protocols then need to distribute the change accurately so clocks stay aligned.
Why they are controversial
Leap seconds are scientifically elegant but operationally awkward. Digital systems can react differently: some step the clock, some smear the second over a longer period, and some fail if they do not expect a 61-second minute. The risk is highest in distributed systems that require tightly ordered timestamps.
The 2035 transition
In 2022, the General Conference on Weights and Measures adopted a resolution on the future development of UTC. The plan is to raise the allowed difference between UTC and UT1 by or before 2035, making continuous UTC possible for at least a century and avoiding regular leap-second insertions.
Why it matters
Leap seconds sit at the boundary between astronomy, metrology, software, navigation, and civil time. Most people never notice them, but they affect satellite systems, time servers, financial records, scientific datasets, operating systems, and any technology that assumes exact global time.