However, by international agreement, the term UTC is recommended for all general timekeeping applications and use of the term GMT is discouraged. This definition was agreed upon in 1967. Atomic clocks based on the cesium atom then became the primary means for accurate timekeeping. A leap second is a second added to Coordinated Universal Time to make it agree with astronomical time to within 0.9 seconds. UTC is an atomic time scale, based on the performance of atomic clocks.
Astronomical time is based on the rate of rotation of the Earth.
Since atomic clocks are more stable than the rate at which the Earth rotates, leap seconds are needed to keep the two time scales in agreement. The first leap second occurred on June 30, 1972. There have been a total of 18 leap seconds to this date. This means that leap seconds occur at a rate of slightly less than one per year. Based on what we know about the Earth's rotation, it is unlikely that we will have a negative leap second in the foreseeable future.