Leap Years and Leap Seconds

Earth takes close to 365.25 days to complete a revolution around the sun. So to align the calendar with the astronomical value, every now and then we do an adjustment. If a year is exactly divisible by 4, but not by 100 unless it's also exactly divisible by 400, then we add February 29th. But when do we add a leap second?
See Also: http://en.wikipedia.org/wiki/Leap_second

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