A. A leap second is a positive or negative
one-second adjustment to the Coordinated
Universal Time (UTC) time scale that
keeps it close to mean solar time. UTC,
which is used as the basis for official
time-of-day radio broadcasts for civil
time, is maintained using extremely
precise atomic clocks. To keep the UTC
time scale close to mean solar time, UTC
is occasionally corrected by an
intercalary adjustment, or
"leap", of one second. Over
long time periods, leap seconds must be
added at an ever increasing rate (see
ΔT). The timing of leap seconds is now
determined by the International Earth
Rotation and Reference Systems Service
(IERS). Leap seconds were determined by
the Bureau International de l'Heure
(BIH) prior to January 1, 1988, when the
IERS assumed that responsibility.

When a positive leap second is added at
23:59:60 UTC, it delays the start of the
following UTC day (at 00:00:00 UTC) by
one second, effectively slowing the UTC
clock
Reason for leap seconds
Leap seconds are necessary partly
because the length of the mean solar day
is very slowly increasing, and partly
because the SI second, when adopted, was
already a little shorter than the
current value of the second of mean
solar time.[1] Time is now measured
using stable atomic clocks (TAI or
International Atomic Time), whereas the
rotation of Earth is much more
variable.
Originally, the second was defined as
1/86400 of a mean solar day (see solar
time) as determined by the rotation of
the Earth around its axis and around the
Sun. By the middle of the 20th century,
it was apparent that the rotation of the
Earth did not provide a sufficiently
uniform time standard and in 1956 the
second was redefined in terms of the
annual orbital revolution of the Earth
around the Sun. In 1967 the second was
redefined, once again, in terms of a
physical property: the oscillations of
an atom of caesium-133, which were
measurable by an atomic clock.[2] But
the solar day becomes 1.7 ms longer
every century due mainly to tidal
friction.

A. The only one I own : SLX,
too hard pick a winner out all of them