The leap second refers to the slowing of the earths rotation (known as UT1) in relation to time. UTC (constant universal time) can be read by the position of the sun but since the advent of the atomic clock (known as TAI) which is for the most part constant from the time it was started, …. Click Here For more information and the 2 clockshttps://www.timeanddate.com/time/leap-seconds-background.html