Network Time Protocol leap second

There are two main reasons that cause leap seconds to occur. The first is that the atomic second defined by comparing cesium clocks to the Ephemeris Time (ET) scale was incorrect, as the duration of the ephemeris second was slightly shorter than the mean solar second and this characteristic was passed along to the atomic second. The second reason for leap seconds is that the speed of the Earth's rotation is not constant. It sometimes speeds up, and sometimes slows down, but when averaged over long intervals the trend indicates that it is gradually slowing. This gradual decrease in the rotational rate is causing the duration of the mean solar second to gradually increase with respect to the atomic second.

Leap seconds are added in order to keep the difference between UTC and astronomical time (UT1) to less than 0.9 seconds. The International Earth Rotation and Reference Systems Service (IERS), measures Earth's rotation and publishes the difference between UT1 and UTC. Usually leap seconds are added when UTC is ahead of UT1 by 0.4 seconds or more.