Can the sunrise/sunset be set somewhere in-game like in the options menu?
The dawn/dusk effect can sometimes be gained/lost by re-installing the game or by installing a patch. There is no in-game option to fix this (you could try re-installing the patch). Edit the '..\Divine Divinity\Run\config.lcl' file in notepad (or any other text editor), add the line "splotchproblem 0" to the end, hit enter (the game expects the last line to be blank) and save. When you start Divinity and load a save, the dawn/dusk effect should be restored.