In the case of Unity .. a google search of "day night cycle unity" came up with the following that may help you view that side of things.
http://twiik.net/articles/simplest-p...cle-in-unity-5
I would suggest you copy the websites content in case when you want to view it at a later date it won't be available.
Other than that the fact that wow has a server time system that they use to change the day/night cycle behind the scenes ( outside of our control as myrroddin said) just demonstrates the source of values and the graphics engine being used will need to be informed how to do work with it. Unfortunately each graphics engine has its own set of functions so hopefully the unity one will give you an idea of which areas can be transferred to other engines.
In theory ( its been awhile since I played with it in unity myself so it may be off a bit ) you would assign an angle range to the horizon ( say -90 in the west and +90 in the east making a total of 360 degrees for the full cycle ). Then assign which angle represents which time ( or how many degrees per second/minute/hour etc ) and update the visual sun/moon light source appropriately making sure to adjust both the colour and intensity accordingly.
There is nothing wrong with asking questions on forums but from experience most programming forums ( which you should be asking these questions on ) expect you to have at least looked into it yourself which includes googling the right word combinations to get the right results. And until you try it out yourself you won't really understand it.