Leap Second

Everyone knows the leap day. Every four years (except century years not divisible by 100), an extra day is added to the year in February to keep the calendar in synch with the seasons.


However, there is also a leap second. This is used every couple of years to keep the Coordinated Universal Time close to the mean solar time. The reason why it is needed is that the Earth's rotation is slowing down, but not by a lot.


The second is added just before midnight on either June 30th or December 31st. The time would go from 23:59:59 to 23:59:60 to 00:00:00. The last time a leap second was added to the UTC was last June 30th.