I think I know what you mean. It takes I think five minutes (or less, but that's what I think) for it to fully transition from day to night and vice versa. As for the clock... the GC has a built in Clock/Calander function and if the Clock reads a night time hour (like 8:00 PM or 3:00 AM), then the menu screen shows the night sky as a backdrop. If the Clock thinks it's the daytime (Noon, 3:00 PM), then it will show the day time sky as the backdrop. If the Clock thinks it's the evening (5:00 PM), then it will show the evening sky as a backdrop.