Dynamic lighting has been a slow project that Sandro has been working towards. But I don't think changing daylight was really on the agenda.
Dawn/Dusk maps would not be too difficult in terms of implementation, but it would take a lot of maintenance work and I'm not sure it would be worth it. The BSP files would swell in size (I think -- an entirely new lightmap will need to be stored for each time setting). And we would also need to adjust all lights in every map to either be on/off in the two new lighting profiles.
Compared to the kinds of battlescape improvements we're working on or still need to implement, it doesn't seem like it's worth the time. These include support for 8+ soldiers, integration of UGVs, new aliens, adding crashed UFOs to existing maps to reduce frequency of UFO Crash map, new maps to complete the storyline, a visibility system, improved alien AI. All of these things strike me as more important and it will be take us years to make progress on all these issues.