If you have a confirmed resort reservation, then you can utilize the 180+10 booking window (book 10 days of meals at 180 days before check-in). Dining plan has zero effect on this. Dining plan only affects whether or not certain restaurants (CRT and Dinner Shows) will charge the full amount ahead of time, or simply require a credit card hold.
The times are 6AM Eastern for Online and 7AM Eastern for phone.