A leap second is inserted in order to adjust the time difference between UTC (Coordinated Universal Time, the international time standard), which is defined by atomic clocks using the frequency of the Cesium atom, and UT1 (UT: Universal Time), which is derived from the Earth's actual rotation. The International Earth Rotation Service (IERS) makes the decision of when to insert a leap second in UTC. Because the Earth's rotational time varies, a leap second is not applied regularly. When applied, the first priority of insertion is given to the end of December and June, and the second priority to those at the end of March and September.
After insertion of a leap second, the time on your watch gets one-second faster than before.
Be sure to adjust the time on the watch.
Signals which radio controlled watches use contain updated leap second information and the watches adjust the time automatically as they receive signals after leap-second insertion.