The clock has several manifestations, controlled by the style sheet used:
Values are generally displayed to a ludicrous precision, the accuracy may be rather less. In particular, values for Sun Right Ascension and Declination may be 10-20 arcsecs out.
For another clock see: An Astronomical Clock
Other web pages I found useful:
An excellent introduction to timescales.
IERS Bulletin A. The very latest data on the rotation of the earth and the relation between UT0, UT1 and UT2.
The relation between apparent and mean sidereal time.
More on sidereal time and the equation of the equinoxes.