Today will seem to be a little longer than usual because an extra second, or “leap” second, will be added at midnight to account for the fact that it is taking Earth longer and longer to complete one full turn—a day—or, technically, a solar day.
Normally, the clock would move from 23:59:59 to 00:00:00 the next day. Instead, at 23:59:59 on June 30, UTC will move to 23:59:60, and then to 00:00:00 on July 1. In practice, this means that clocks in many systems will be turned off for one second.
The International Earth Rotation and Reference System Service (IERS) observes the Earth’s rotation and compares it to atomic time. When the difference between the two approaches 0.9 seconds, they order a leap second to be added worldwide.
The most recent leap second was inserted into the atomic time scale on New Year’s Eve of 2008. The 2012 leap second is the 35th leap second to be added and the first since 2008.
For more information, visit http://wwp.greenwichmeantime.com/info/leap-second.htm