Do you know that occasionally at around 23:59:59 UTC, the minute is going to have an extra second, i.e, a total of 61 seconds? Yes you heard it right. There are overall 27 such seconds inserted in the past. When the correction is made, it usually happens at the end of June or December or sometimes both.

A day on earth is typically not exactly 24 hours long. Earth usually takes about 23:59:54 seconds to complete one rotation about its own axis. Because of the Moon’s gravitational pull, Earth’s rotation slows down slightly, which in turn results in a slightly longer day. If you take the average over 365 days, the length of the day is computed to be 24 hours. Therefore, we consider length of the day to be 24 hours. However, occasionally this average value need slight correction to accommodate slowing down of Earth’s rotation. Therefore, International Earth Rotation and Reference Systems Service (IERS), adds an extra second on some occasions to rectify the error in length of the day. This extra second is known as leap second.