in an extremely chaotic system, we all get to make seemingly arbitrary changes to our clocks. Such a change is coming at the end of this year.
Leap seconds occur on average every 2-3 years, due to the long-term slowing of the earth's rotation and the canonical second being based on astronomical observations that calculated an extremely accurate value - for the late 19th century "second". Oops. The presence or absence of a leap second is announced toward the beginning of January or July for the end of the following June or December, though provisions are made to allow "emergency" leap seconds being introduced at the end of March or September. They are decided by the International Earth Rotation Service (recently renamed the International Earth Rotation and Reference Systems Service, but still known as the IERS.)