earth’s rotation is slowing, and a day is actually now about 86,400.001 seconds long. That extra millisecond means that after a thousand days or so our atomic clocks are running a second ahead of the earth’s rotation, so an extra second is inserted on a chosen day to sync our clocks back up with the earth. A leap second is typically inserted on either June 30th or December 31st at the end of the day, UTC time. The last minute on those days can have 61 seconds if the need is determined by an international astronomy society. They send out a notice about 6 months prior, and have just done so.