NavList:

Is Aries ever ahead of or behind schedule? Yes, but it's a tiny thing -- just a fraction of a second. The Earth is slowing down, and the standard (SI) second was defined based on the mean rotation of the Earth in the late 19th century. Because of this our modern clocks run a wee bit too fast, and every now and then we have to "stop" them for one second to allow the spinning Earth to catch up. This "stop" is accomplished by adding an extra second to the last minute of the day on December 31 or occasionally June 30. This is called a leap second. They are inserted every couple of years on average, but the schedule is unpredictable. Note that this adjustment to time only occurs in whole second increments so the location of Aries relative to longitude can still be out of perfect alignment by some fraction of a second. These adjustments are irrelevant to celestial navigation, but if leap seconds are ever dropped from the time-keeping standard (which is desirable for various reasons), then it might be necessary to adjust UTX (whatever Universal Time might be called) to correct for this in decades ahead.

Frank Reed