The Earth orbits the Sun, and each orbit equals one calendar year. The Earth also revolves around its own axis, and each revolution equals one day. To remove the discrepancies between atomic and astronomical time, we change the calendar, as needed.

Once every four years, we add one full day to the calendar. Since February is the shortest month, with only 28 days, we add the day there - February 29. Each year, we also change the clocks either forward or backward by a single second. This happens on June 30 and December 31, and it is done when the times don't align properly.

The determination as to whether such an adjustment is necessary falls to the International Earth Rotation Service of the International Bureau of Weights and Measures in Paris. The announcement to insert a leap second is given whenever the difference between the two time clocks approaches one-half second; after UTC (Universal Time Code) 23:59:59, a positive leap second is inserted at 23:59:60, that is, before the clock indicates 00:00:00 of the next day. Negative leap seconds are also possible, should the Earth's rotation becomes slightly faster; in that case, 23:59:58 would be followed by 00:00:00.