Calendars and time
Leap Second
Direct answer
Leap Second: A Leap Second is a one-second adjustment that may be inserted into or removed from UTC to keep it close to UT1.
A Leap Second adjusts UTC by one second relative to Earth rotation.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- UTC Offset, Delta T, Calendar, Julian Date
A Leap Second adjusts UTC by one second relative to Earth rotation.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Leap Second
Leap seconds became part of UTC in 1972 under international timekeeping arrangements.
Why Leap Second matters
Atomic seconds are uniform while Earth rotation varies, so UTC policy has used occasional adjustments to limit their separation.
Leap Second in practice: an example
A positive leap second can label an extra instant 23:59:60 UTC at an authorized month end.
Where readings of Leap Second go wrong
It is a mistake to predict a leap second from a calendar pattern; announcements depend on measured Earth rotation and current international policy.
Sources and editorial method
Research references used for this entry include:
- BIPM SI Brochure Annex: Time - International metrology reference for the SI second, TAI, UTC, and Terrestrial Time.
- IERS Conventions (2010), Technical Note 36 - International standards for Earth rotation angle, reference systems, time arguments, and Earth-orientation data.
- NIST: UTC(NIST) Time Scale - Official description of UTC realization, atomic time, and leap-second coordination.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
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Common questions
What is a Leap Second?
A Leap Second is a one-second adjustment that may be inserted into or removed from UTC to keep it close to UT1.
What do people get wrong about a Leap Second?
It is a mistake to predict a leap second from a calendar pattern; announcements depend on measured Earth rotation and current international policy.
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