If you have just eighteen hours per semester to graduate by that date then you should apply the year before your graduation date. If you have alot of hours left to complete then the dental schools will not want to gamble on your ability to complete them. I applied early and I had 39 hours left in my last two semesters, I did it but the dental school was not sure if I would and was not willing to reserve a spot for someone who might not be able to attend.