Looks like some bad GI issues happening in your render there. Weird, haven't seen anything like that for ages in Terragen.
17 hours is an awfully long time, as well. Have you upped the quality settings significantly? What are your computer spec's? I am only using a 2ghz dual core here and the post above yours rendered in about 30-odd minutes. The cloud 'acceleration cache' has been enabled for that one which makes it much quicker but still, I think I could safely say that even with no acceleration it would have taken maybe only 2-3 times as long.
You do have a very thick atmosphere that I would blame for the time increase, if it had lots of samples but, it still appears quite grainy so, I've no idea why it would be taking so long.
The red sky sunset example above took about 5 hours with no acceleration, that's pretty long in my book for such a render.