Leap Second

leap second: an extra second inserted into UTC at the end of some years between 24:00:00 Dec 31 and 00:00:00 Jan 1 to ensure that UTC remains in step with the Earth's diurnal rotation. It may also be added at the end of June.

Leap Second
A second (see Second, Système International) added between 60s and 0s at announced times to keep UTC within 0s.90 of UT1. Generally, leap seconds are added at the end of June or December.
Least Action ...

Leap Second -- A second which may be added or subtracted to adjust UTC at either, both, or neither, of two specific opportunities each year.

Leap seconds and other aspects
Since 1972, some years may also contain one or more leap second
Leap second ...

Leap Second
Light Pollution Facts
Light Pollution -- International Dark-Sky Association ...

GMT (Space Flight Glossary - JPL) Greenwich Mean Time, similar to UTC but not updated with leap seconds. GMT (abbr) (NASA SP-7, 1965) = Greenwich mean time.

Because the rate of the earth's rotation is gradually slowing, it is occasionally necessary to add an extra second, called the leap second, to the length of the UTC year; ...

Coordinated Universal Time (UTC) is the modern standard, and takes into account leap seconds. In popular usage, the two terms are synonymous.
Abbreviation: GMT
See also: Coordinated Universal Time
eastern ...

(But some years may contain leap seconds.)
See also common year starting on Sunday and dominical letter.

7 seconds per year since (see leap seconds). International Atomic Time (TAI) was set equal to UT2 at 1 January 1958 0:00:00 . At that time, Î"T was already about 32.18 seconds.

Atomic time was officially adopted 1972 January 1. From 1972 January to 1974 January 1, 3 leap seconds had to be introduced to keep atomic time within 0.7 seconds of Universal Time. [H76]
Atomic Weight ...

The name "MTC" is intended to parallel the Terran Coordinated Universal Time (UTC), but this is somewhat misleading: what distinguishes UTC from other forms of UT is its leap seconds, but MTC does not use any such scheme.

a longer explanation of the tides and their causes and effects
leap seconds and the slowing of the Earth's rotation
historical info about observations of the Moon
Lunar Meteorites
Moon Phases explained
The impact theory of lunar formation ...

small difference accumulates every day, which leads to an increasing difference between our clock time (Universal Time) on the one hand, and Atomic Time and Ephemeris Time on the other hand: see Î"T. This makes it necessary to insert a leap second at ...

See also: Second, Time, Earth, Solar, Rotation