A billion years ago a year was 385 Earth days, too. So the days were shorter and the years were longer.
Over time Earth rotation slows down and we have to add time to the clock. When we do that the time we add are called “leap seconds.” This takes place at midnight UTC time where they have a 61 second minute in order to coordinate world clocks with the ever slowing rotation of Earth. This happened December 31, 2016 at 23:59:60. Notice that 23:59:59 should click over to 00:00:00, but in this case an extra second was added and the time became 23:59:60 before it clicked over. All leap seconds are added either June 30 or December 31.
This happens only occasionally. Nothing to worry about.