Larian has said that you could create day and night through triggers to change the lighting.
Making a duplicate map though, is not a very efficient plan. Each map in D:OS has a buttload of things to interact with, move around, destroy, and the state of each and every one of them is saved. That's why the saved games are so big. So, you would need to duplicate the current state of the map, which probably means a lengthy load time. And that's also hoping that duplicating the map doesn't also break all the intertwined story and script triggers.
Plus, because of multiplayer, you'd have to put this trigger in a separate map than the one you want to change the time of so it doesn't change suddenly when player 2 is wandering about in the middle.
