Saturday (June 30) will be one second longer as a "Leap Secomd" is added to the Civil Time scale, to compensate for the slowing rotation rate of the Earth. The second will be added at the end of the day, Coordinated Universal Time (23:59:60), which translates to 7:59:60 p.m. Eastern Daylight Saving Time.
Civil Time is now controlled by atomic clocks, which do not compensate for the slowing of the Earth's rotation rate. Hence, since 1972, scientists have added 25 (including 2012) positive leap seconds, to keep clocks synchronized with UT1, Astronomical Time also known as Mean Solar Time.
MORE INFORMATION --
The Science Behind Saturday's Leap Second (LiveScience.com):
Leap second (Wikipedia):
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