Glad that you found a way, let me share a way that could help.
First, how long it takes to go from sunny to fully dark? In my example, lets say 2 hours.
So, at 5pm it will start to go darker till 7pm when it is fully dark. (From 0 to 100% opacity).
I would do something like this:
On 5pm set stage to -> sunset
Is stage = sunset and sprite opacity is not equal to 100%:
Every 1 second set sprite opacity to self.opacity + 0.83 (0.83 because we have 2 hours in 120 seconds, so 100/120).
What will happen is: at 5pm it will start to change the opacity 0.83% every second till reach 100%. You can also do the same for sunrise, inverting the opacity. If you want less time or more time on the process, just calc 100/seconds you want.