Sophie Bushwick in Scientific American:
NIST-F1 is one of several international atomic clocks used to define international civil time (dubbed Coordinated Universal Time, or UTC), a job they perform a little too well. In fact, atomic clocks are actually more stable than Earth's orbit—to keep clocks here synched up with the motion of celestial bodies, timekeepers have to add leap seconds. The use of a leap year, adding a day to February every four years, locks the seasons, which result from Earth's orbit about the sun and the planet's tilt as it orbits, into set places in the civil calendar. Similarly, leap seconds ensure that the time it takes Earth to spin 360 degrees is equal to one day as defined by humans and their atomic clocks. Most recently, an extra second was tacked on to universal time on December 31, 2008.
However, since 1999, the Radiocommunication Sector of the ITU has been proposing the elimination of leap seconds from the measurement of UTC. Although the organization did not participate in the creation of the current leap second system, the radio waves it regulates are used to transmit UTC, giving it some influence.
Getting rid of leap seconds would certainly make it easier to calculate UTC, but this measure would also decouple astronomical time from civil time: The time measured by atomic clocks would gradually diverge from the time counted out by the movement of Earth through space. Eventually, one year will no longer be the length of Earth's orbit around the sun. Instead, it will be equivalent to a certain number of cycles of radiation from the cesium-133 atom (almost a billion billion cycles, to be precise).