The International Earth Rotation Service (IERS) just announced that a leap second will be added at the end of June 30.
Since their introduction in 1972 we have gained 15 seconds and lost none.
Scienetest at the IERS monitor the rotation of the earth. Atomic clocks keep much better time than the Earth itself. Earthquakes, tidal drag, and the weather all affect the rotation of the Earth.
The last leap second was added in 2012 and caused a few popular website servers to crash when the leap second ticked unexpectedly into place.