The Royal Observatory is also in charge to inform the Belgian users about the leap seconds which are applied, if necessary (i.e. if the difference between UTC and the Earth rotation comes close to 0.9 sec) on June 30 and/or December 31 of the year considered. These leap seconds are due firstly to the bias associated with the definition of the atomic second (slightly shorter than the second determined from the present rotation speed), and secondly to the variations of the Earth rotation speed. The TAI is a very stable time, while the Earth rotation rate is not constant, due to energy exchanges between the atmosphere and the solid Earth, as well as between the liquid inner core and the solid part, called mantle. The IERS (International Earth Rotation Service) determines permanently the difference between the UTC and the time based on the Earth rotation. The IERS decides if a leap second is necessary in order to keep this difference below 0.9 sec. The combination of TAI and the leap seconds gives rise to the UTC which is close to the Earth rotation, and which is used by peoples in all the world.