The timescale can be set freely from within scripts. So you could write a script which changes timescale according to your needs, e.g. by entering the wanted timescale via keyboard.
For the observation of a planet's rotation etc. this won't really work. There is no fixed time increment because the time needed to render a frame varies - if Celestia takes 20ms to render a frame, the simulation time will advance 20ms * timescale.
But you can increment simulation time by a fixed amount using a script (surprised?). In fact, there is already such a script available at my
Celestia-page, it's called
timestepkeys_v1.1.celx.
It works by retrieving daylength etc. of the currently selected planet from a table (i.e. this only works with planets in our solarsystem), and then changing the time once per frame by one either one year (press "a"), one sideral day (press "s") or one solar day ("d"). Press shift and any of these keys to decrement time.
Harald