Making a Day & Night Cycle?

How exactly would one be able to make a day & night cycle?

To clarify a bit, is it possible to:
[ul]Make the skybox texture change from a day time texture, to an evening texture, to a night texture, and so on, either automatically or with the help of a code or script (a lua script in my case).[/ul]
[ul]Make the light_environment change automatically, with the skybox texture change, or with the help of a code or script (a lua script in my case)[/ul]
[ul]Make certain entities change values depending on the “time of day” (although I’m fairly certain it’d probably be easy to do this if the previously-mentioned events are possible).[/ul]

And, in addition, how much lag would these cause to clients and server hosts?

I believe there was a thread a while back, that looked like this:
http://www.facepunch.com/showthread.php?t=298007

Its somewhere in there.