Did you know that there are leap seconds?
The Earth is constantly undergoing a deceleration caused by the braking action of the tides. Through the use of ancient observations of eclipses, it is possible to determine the deceleration of the Earth to be roughly 1-3 milliseconds per day per century. This is an effect which causes the Earth's rotational time to slow with respect to the atomic clock time. Since it has been nearly 1 century since the defining epoch (i.e. the ninety year difference between 1990 and 1900), the difference is roughly 2 milliseconds per day. Other factors also affect the Earth, some in unpredictable ways, so that it is necessary to monitor the Earth's rotation continuously.
In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a leap second is added to the atomic time to decrease the difference between the two. This leap second can be either positive or negative depending on the Earth's rotation. Since the first leap second in 1972, all leap seconds have been positive. This reflects the general slowing trend of the Earth due to tidal braking.
I should read more.