At the third stroke, the time will be 11:59 and 60 seconds precisely...
|Every second counts.|
To explain the reasoning behind leap seconds, without getting too scientific, the speed of Earth's rotation varies by tiny amounts due to geological occurences, as well as the pull of the Moon and the Sun, so the leap second is basically a way of ensuring our calendar remains precisely accurate.
You may think, "It's only a second! What's the big deal about a second?" but in actual fact a surprisingly high amount of activity can take place in a single second.
For example, every second...
1) ...the Earth orbits 18.5 miles around the Sun!
2) ...the Solar System travels 155 miles around the galactic centre of the Milky Way!