On June 30 2012 at 23:59:60 UTC a leap second will make the day one second longer. The leap second is inserted between 23:59:59 and 00:00:00 UTC to compensate for variations in the rotation of the Earth.
Most clocks will just ignore the leap second, but Time.is will count all the way up to 60 seconds!
The leap second does not occur on June 30 everywhere! The date and time depends on your time zone. In New York the leap second will occur at 07:59PM:60 on Saturday, June 30, 2012. To check other locations, use the search box above.
Remember to set your clock one second back after the leap second!