Leap second

A one-second adjustment to Coordinated Universal Time (UTC) that keeps civil time closely aligned with Earth’s irregular rotation.

Coordinated Universal Time: The international time standard formed from atomic clocks and adjusted to remain close to Earth’s rotation. UTC is the time scale whose seconds are lengthened or repeated when a leap second is inserted.
Atomic clock: A clock that measures time using the characteristic frequency of atoms or ions. Atomic clocks made uniform timekeeping precise enough for rotational drift to become noticeable.
Network Time Protocol: A protocol for synchronizing computer clocks over packet-switched networks. NTP distributes UTC time and must handle leap-second announcements and adjustments.
Leap smear: A technique that spreads a leap-second adjustment gradually across a period rather than inserting one abrupt second. It avoids a single repeated or skipped timestamp but temporarily makes the displayed time differ from UTC.
Universal Time: A time scale based on Earth’s rotation, determined from observations of the meridian passage of celestial objects. The difference between this rotational time and atomic time determines when a leap second is needed.
Quartz clock: A clock that keeps time by applying an electric field to make a quartz crystal oscillate. Quartz clocks helped expand precise timekeeping before atomic time became the basis of UTC.
Precision Time Protocol: A protocol that synchronizes clocks in networked measurement and control systems with high precision. Its systems may need coordinated handling to avoid timing errors at a leap second.
Leap minute: A proposed timekeeping adjustment that would add an entire minute to UTC when needed to track Earth’s rotation. It was discussed as an alternative to frequent one-second corrections.
Earth rotation angle: An angle specifying Earth’s orientation in space, used to describe its rotation relative to celestial reference frames. Monitoring Earth’s orientation reveals the drift that leap seconds are intended to limit.
International Telecommunication Union: A United Nations agency that coordinates international telecommunications and radio-frequency standards. Its conventions have governed the definition and coordination of leap seconds in UTC.

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