I want to set the "time adjustment" in my preferences to "-8.5 hours" (the correct local time here in South Australia), but it seems that such a thing is impossible--I can have -8 hours or -9 hours but nothing in between. Have I overlooked something obvious?

(Apologies if this is a dumb newbie question, or if I'm asking in the wrong place--I've spent some time searching and haven't managed to find an answer. Thanks for any help you can offer.)