A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time(UTC) in order to keep its time of day close to the mean solar time. The most recent leap second was inserted on June 30, 2012 at 23:59:60 UTC.

The UTC time standard, which is widely used for international timekeeping and as the reference for civil time in most countries, uses the international system (SI) definition of the second, based on atomic clocks. Like most time standards, UTC defines a grouping of seconds into minutes, hours, days, months, and years. However, the duration of one mean solar day is slightly longer than 24 hours (86400 SI seconds). Therefore, if the UTC day were defined as precisely 86400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. The purpose of a leap second is to compensate for this drift, by scheduling days with 86401 or 86399 SI seconds.

Because the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), when needed to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 second. Between their adoption in 1972 and June 2012, 25 leap seconds have been scheduled, all positive.

http://en.wikipedia.org/wiki/Leap_second

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Gus: one extra second did not help with my schedule of doing too much with too little time... but that's life... Note that if you are travelling by ship, plane or roller skates — a one second time difference can mean an error of one sixtieth of a nautical mile or about 31 metres... If you are "doing war", it can mean hitting the target one second too late or 31 metres to the left if your GPS is skewed with time...