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Saturday, 30 June 2012, was one second longer than usual, since an extra second was inserted between 23:59:59 and 00:00:00. Therefore, the clocks moved from 23:59:59 to 23:59:60, instead of 00:00:00. This leap second was introduced to reconcile between the Coordinated Universal Time (UTC), the international time standard, which is based on atomic clocks, and the astronomical Universal Time (UT1), which is based on the rotation of Earth about its axis.
On 30 June 2012, a leap second was added to clocks, making that day one second longer than normal days. Typically, every day, the clock moves from 23:59:59 to 00:00:00, the start of the next day, but, at 23:59:59, on 30 June, clocks moved from 23:59:59 to 23:59:60, and then to 00:00:00, on 1 July.
A leap second is a one-second adjustment that is occasionally inserted to Coordinated Universal Time (UTC), the international time standard, by which the world regulates time and clocks, and is based on highly precise atomic clocks. The purpose of introducing leap seconds is to adjust the difference between UTC and the astronomical Universal Time (UT1), which is based on measuring the rotation of Earth on its axis.
Scientists can precisely measure and monitor Earth’s rotation period, by applying sophisticated techniques, including observations of remote galaxies with radio telescopes. Data acquired by radio telescopes have demonstrated that Earth is not rotating regularly, but its rotation period is very slightly and slowly increasing, due to the influence of the Moon’s gravity. Roughly, every century, the day gets about 0.0014 second longer, leading to a discrepancy between UTC and UT1.
The 30 June extra second is the 35th leap second to be added and the first since 2008.
References:
NASA
www.nasa.gov/
Wikipedia