Set Lighting according to Time of the day

I am creating a simple Graphics Engine and I want to let the user set the time of the day for the scene, but I have problems translating it into the lighting direction. The light direction is now given by a vector of three float. i.e. The user set time = 12 (noon). I want the light direction to be right on the top of the scene. Is there a way to achieve that? Thank you