Some gamemodes have day/night cycles built in with them, heres a day/night cycle that is standalone and can be used in any game mode.
Provides a synchronised day/night cycle in the Onset world. The length of a 24 hour period can be specified in seconds in DAY_LENGTH_IN_SECONDS in daycycle.server.lua - default is 60 seconds.
I plan to do some stuff, I threw this together on my lunch break at work so its basic right now.