Believe it's on a rolling 180 day release. Hours will definitely be out long before your 60 day mark.
However hours are usually updated about a month out. So you'll have to check as the dates get closer.
Usually it's to extend hours. This is why making a PPO ADR for 8 during a 9am opening might bite you because they might change park opening to 8. Usually emh are already locked in but they can still move around. And they can change anything, but normally they won't take away hours. Little more rare if they change an 8am opening to 9am