Didn't really look hard but If anybody wants to mess with day/night codes, at least part of it hangs around this area of the code, probably want to confirm that npcs and monster spawn if it is changed, but I do know that freezing values can keep the day from advancing visually.
CalctAndSetTimeOfDay that sets TimeOfDay to one of 0 = Morning, 1 = Day , 3 = Evening, 4 = Night
For weather, didn't really search much at all, but the environment files seems to indicate it is 0 = Clear, 1 = Cloudy, 2 = Thunderclouds, 3 = Rain, 4 = Thunder Storm, 5 = After Rain, 6= Snow , but couldn't really find those values but maybe I had cheat engine configured wrong or they are part of a bit field, they are wrong, or don't know.