leap second

A leap second is a second that is added to a specified day, to keep the calendar in close alignment with international atomic time. (Atomic time is based on the extremely regular vibrations that occur within atoms.) The reason why leap seconds are periodically needed is that the Earth's rotation is very gradually slowing down. One leap second was inserted on June 30, 1992. It was accomplished as follows: 23 hours 59 minutes 59 seconds universal time was followed by 23 hours 59 minutes 60 seconds. This, in turn, was followed by 0 hours 0 minutes 0 seconds on July 1.