Oooh, that's odd. Your setDayCycleTime conflicts with the inc/dec time method. Basically, if you set the time, then increase it, it jumps back to where it was before the set time (I think). Is it really moving the offset, or using another method?
Edit: Just tested, your method doesn't move the Offset, and then when the Inc/Dec changes it, the sun resets. I would just change the Inc/Dec to your method, but that wouldn't solve the thing about users changing the slider, and having the sun jump majorly after setting the time. I think we're back to stage one :/