Dining times are always around 6.15 and 8.30. However on European itineraries they have been known to do staggered dining within each seating as the Europeans like to eat later than the Americans. On Voyager last year dining times were 6.30/7.00 and then 9/9.30. Basically the simple answer is you will not know the exact times until you board.
6.30 and 8.30 pm
I wouldnt suggest the early sittings as it curtails your time ashore or on deck... unless you fancy the early evening sessions in the casino, as there would be little else to do after eating.