Mainly due to the slowing down of earth rotation, it is about time to add another (the 35th) leap second to UTC in order to keep its time of day close to the mean solar time. NASA features an explanation of why the leap seconds have to be inserted, how earth rotation can be measured precisely and why it is impossible to give precise predictions on when the next one will happen.
The length of day is influenced by many factors, mainly the atmosphere over periods less than a year. Our seasonal and daily weather variations can affect the length of day by a few milliseconds over a year. Other contributors to this variation include dynamics of the Earth's inner core (over long time periods), variations in the atmosphere and oceans, groundwater, and ice storage (over time periods of months to decades), and oceanic and atmospheric tides. Atmospheric variations due to El NiƱo can cause Earth's rotation to slow down, increasing the length of day by as much as 1 millisecond, or a thousandth of a second.
VLBI [Very Long Baseline Interferometry] tracks these short- and long-term variations by using global networks of stations to observe astronomical objects called quasars. The quasars serve as reference points that are essentially motionless because they are located billions of light years from Earth. Because the observing stations are spread out across the globe, the signal from a quasar will take longer to reach some stations than others. Scientists can use the small differences in arrival time to determine detailed information about the exact positions of the observing stations, Earth's rotation rate, and our planet's orientation in space.
(Score: 0) by Anonymous Coward on Tuesday June 30 2015, @07:54AM
From the ntpdate man page: [unix.com]