A leap second is an additional second added to Coordinated Universal Time (UTC) to keep it in sync with the Earth's rotation. The Earth's rotation is not perfectly constant, and it can vary due to factors like gravitational forces and seismic activity. To ensure that our clocks remain accurate, leap seconds are occasionally inserted, usually at the end of June or December. Since the introduction of leap seconds in 1972, there have been 27 adjustments. The decision to add a leap second is made by the International Earth Rotation and Reference Systems Service (IERS), which monitors the Earth's rotation and timekeeping standards.