Nothing is perfect, except for the perfect things. It does not take a precise 24 hours and zero minutes and zero seconds for the earth to complete one spin on its axis; it takes slightly longer, but not so much longer that you could even call it a “tick.”
The earth’s rotation each day is only a tiny fraction of a millisecond slower than what we otherwise call a day, but these partial seconds can eventually add up. Twenty-six times since 1972, the international bureau of standards that handles time issues has added a “leap second” to all of our lives. The last year with a leap second was 2015, so if that year felt longer for you, there is a reason: It was. By one second. Clocks everywhere could have read “11:59:60” at midnight the night of the leap second, but they did not because no one makes clocks that do that.
The next leap second will delay the arrival of 2017 immediately before midnight on December 31.