(Please excuse my English)
A well known fact about Earth's rotation(around its axis) is that the rotation is slow down due to the tidal friction. And because of it, the length of Day is slowly increasing.
Originally, the definition of Second(time) was derived from the day length and Earth's rotation. The original definition of the second was 1 day / 24 / 60 / 60 = 1 day / 86400. However, the modern definition of the second is derived from the atom's oscillation period, for precise time keeping, independent of Earth's rotation, because of irregularity of Earth's rotation. Thus, compared to the original definition, the modern definition of the second has rigidly fixed value.
My question is : Someday in future, the length of a day will be longer than 86400 atomic seconds. Then what should future human do?
Changing the definition of the second will lead to incredibly complex situations. Also changing the definition of the day will cause tremendous complex situation.