Even before the advent of clocks and watches, humans gauged events using rotations of the Earth. The foundation of timekeeping is based on the number of times the planet turns within an orbital circuit even if it was originally expressed as the number of days in a year. The ancient astronomers and ,in particular one brilliant man in antiquity discovered that a year can't be based on a consistent 365 day cycle but an additional day is required to maintain rotations to the fixed orbital points. This day/rotation is now February 29th and forms an inviolate proportion which references rotations to orbital circumferences.

When clocks emerged, the 24 hour day was based on the average noon cycle within the same 1461 day framework, again, observations are gauged using rotations with the appearance of the Sun followed by the appearance of the stars - this alone constitutes the only meaningful description of a day astronomically and for the purpose of terrestrial sciences.

The unwarranted creation of a 'solar day' and a 'sidereal day' completely obliterates the usage of the background stars and the central Sun as a reference for the orbital motion of the Earth. By using one specific astronomical event when a star re-appears from behind the glare of the Sun to determine the orbital position of the Earth,humanity was able to create a gauge using rotations to define the number of times the Earth completes a circuit of the Sun to a close proximity.

The 'leap second' is an extension of the 'solar vs sidereal' framework which still creates the notion that the planet turns 366 1/4 times per orbital circuit and it is exceptionally painful to witness for any rational person -

"During one orbit around the Sun, Earth rotates about its own axis 366.26 times" Wikipedia

http://en.wikipedia.org/wiki/Earth