Every once in a while, to account for the gradual slowing of the earth's rotation, a "leap second" is added to the clock in the form of an extra second at midnight by the International Earth Rotation Service (yes, this is a real thing:
https://en.wikipedia.org/wiki/IERS ). The effect on ordinary clocks is to basically make the clock stop for one second to let the slowing earth "catch up" with Greenwich Mean Time.