This is already ingame...
When making a map you can implement transitions day-night, different lightsettings , start rain, etc...
I always used this when making maps for coh1, it gives maps a realy nice touch.
Starting at noon slow transition into evening at 20-25 into dark night at about 1 hour, for example.
edit: it´s weird noone utilizes this, it´s such a nice feature to have on a map
Yeah; I know...it's a nice cosmetic addon for maps, but really adds nothing to gameplay value. I'd like to see reworked game mechanism for night time fights (like reduced vision + accuracy).