A leap second is the one-second adjustment that is periodically applied to the atomic clocks that act as reference points for world's timekeeping in order to keep them in sync with the Earth's rotation. One such time stadard is the Coordinated Universal Time (UTC), which is widely used for international timekeeping in most countries. Like most time standards, UTC uses the atomic clock seconds to form minutes, hours, days, months, and years. However, the duration of one mean solar day is slightly longer than 24 hours (86400 atomic seconds). Therefore, if the UTC day were defined as precisely atomic 86400 seconds, the UTC time-of-day would slowly drift apart from that of solar-based time reference. The purpose of a leap second is to compensate for this drift, by scheduling days with 86401 or 86399 atomic seconds.

On June 30, 2012, a leap second was added to UTC at 23:59:60.