Don't forget: Set A-clocks back a second June 30
A leap second will be added to the world's time standard to keep clocks in step with solar time. Leap seconds, used since 1972, are needed because the time standard (called UTC, for coordinated universal time) is kept by atomic clocks, which run slightly faster than solar time.
In laboratories around the world, atomic clocks are nearly a million times as constant in rate as the Earth's spin, providing more accurate standards for keeping time.
In 1980, no leap second was needed because the two time scales did not diverge as much as in previous years. But by the end of 1981, it is projected that UTC and solar time will be over a second apart. Instead of waiting until December, the International Time Bureau decided to order the leap second at 23: 59:60 UTC June 30 (just before 8 p.m. Eastern daylight time), making June 30 o ne second longer than normal days.