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Leap Seconds: Why Clocks Sometimes Need an Extra Second

Leap seconds are occasional one-second adjustments that keep Coordinated Universal Time close to time based on Earth’s irregular rotation.

Timeline

  1. Continuous measurement: IERS observes Earth orientation and tracks the difference between rotational time UT1 and Coordinated Universal Time.
  2. Announcement: Under the present system, IERS announces a needed adjustment in advance, normally for the end of June or December.
  3. Future transition: International metrology and telecommunications bodies are developing implementation details for a larger permitted UT1–UTC difference in or before 2035.

Modern civil time joins two different ideas. International Atomic Time uses highly stable atomic-clock measurements, while UT1 follows Earth’s angle of rotation relative to the sky. Earth does not rotate at a perfectly uniform rate: atmosphere, oceans, the fluid interior and longer-term tidal effects all contribute to variation. Coordinated Universal Time, or UTC, runs at the atomic rate but has used whole-second adjustments so that it stays close to UT1 and therefore to mean solar time. [1][2][3][4]

A leap second is that adjustment. Under the present rule, one is applied when the measured and predicted difference between UT1 and UTC approaches the allowed limit of 0.9 second. The International Earth Rotation and Reference Systems Service monitors the difference and issues official bulletins. The decision is observational, not calendar arithmetic, so leap seconds do not arrive every fixed number of months and are unrelated to the rule that adds February 29 in a leap year. [1][2][3][4]

Every leap second used so far has been positive. At the end of the selected UTC day, the displayed sequence can run 23:59:59, 23:59:60, then 00:00:00. Standards also provide for a negative leap second, which would remove a second if Earth’s rotation made that necessary, but none has yet occurred. Adjustments have normally been scheduled at the end of June or December after advance notice; software should consume an authoritative announcement rather than try to predict one from a historical average. [1][2][3]

The adjustment is small for a person but awkward for computers. Many data formats and programs assume every minute has exactly 60 seconds, and distributed systems can disagree about how to represent or spread the extra second. BIPM identifies risks for navigation, telecommunications, energy transmission and other synchronized infrastructure when operators use different mitigation methods. A leap smear, in which a provider gradually changes clock rate, is an implementation choice rather than the standardized UTC step and can temporarily disagree with systems using another method. [1][4][5]

International policy is moving toward a more continuous UTC. Resolution 4 of the 2022 General Conference on Weights and Measures decided that the maximum permitted value of UT1 minus UTC will be increased in or before 2035, with a plan intended to keep UTC continuous for at least a century. That decision is often summarized as “ending leap seconds,” but the official resolution leaves the new maximum value and detailed implementation to further coordination. It does not mean atomic time and Earth rotation become identical. [4][5]

Without frequent one-second steps, UTC and UT1 will be allowed to separate by more than today’s limit. Most clocks and civil uses would continue to use UTC, while astronomy, satellite operations and other applications needing Earth-rotation angle can use measured UT1 information or published corrections. The practical effect depends on the final international specification and national or industry implementation, so a precise switchover claim should be checked against current BIPM, ITU and IERS notices. [2][3][4][5]

For ordinary users, no manual clock action is normally required; operating systems, network time services and regulated broadcasters handle official time information. Engineers should test timestamp parsing, ordering, logging and distributed consensus against both positive and negative discontinuities and should document whether a service steps, smears or ignores an adjustment. For any future event, use IERS bulletins and a national metrology institute rather than a viral countdown. The durable lesson is that a leap second reconciles atomic time with a rotating planet, while the policy for doing so remains under active international governance. [1][2][3][4]

Sources

  1. NIST — Leap Seconds FAQs
  2. NIST — Leap Second and UT1-UTC Information
  3. IERS — Leap Seconds
  4. BIPM — Resolution 4 of the 27th CGPM (2022)
  5. BIPM — Recent Insights into Earth Rotation and Timekeeping

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