Jamie has given you the best and most comprehensive answer so far, and as I mentioned earlier, you won't know exactly what the time is until you get on board, but all you know right now is that you are on the 'early' sitting, rather than the 'late' sitting, so if you are happy to eat late but unhappy to eat early you might want to change that, but otherwise once you get the time in your cabin you can still request a change...