I have my serverTimeAcceleration set to 6, meaning 6 hours of in-game time should pass in 1 hour of real time. I've got the date set so that the server starts getting dark by 7pm and then the sun's coming up around 4am. With the serverTimeAcceleration setting of 6 and darkness lasting roughly 9 hours, the players should have over 1 hour of real time in the dark, but the darkness only lasts about 20 minutes. I determined the sunset and sunrise times by using an admin tool on my server to change the time until I found the time that sunset and sunrise occur.

So, does serverTimeAcceleration really work correctly? With it set to 6, there's no reason the darkness should only last 20 minutes.