What exactly you don't understand?
You need the Time - a counter, a global value - that shows you, whats the time.
Then you have to say, that when it's between 6am and 6pm it's day, between 6pm and 6am it's night.
For day - or better say for the night you can use layout-effects or an dark, semitransparent layer, that will shown, when it's night.
Thats the idea of day and night circle!
But it would be interesting, how you did it now?