A day cycle is a sequence of sky settings. Each sky setting specifies a position for the sun. In the simulator, we also have a "sun position", which can be read using the LSL function llGetSunPosition; this is, for example, how SL sundials, clocks, and auto-sensing lights work (the lights go on when the estate time says the sun is below the horizon).
I think it's entirely correct that one should be able to create "days" in which the visible sun takes some odd course through the sky, including looping back and forth, etc: we can do this now.
What we can't very easily do is create a day in which the sun actually tracks the position that is returned by llGetSunPosition. It should be possible to specify that for a given sky setting, the sun position should be governed by the SL time.