I'm really shocked no one has answered your question yet. I know a lot of working adults play ffxiv, so I would think someone would know. But oh well, i'll tell you.
Normal business practice is that you are charged for the full month. Some business will pro-rate you if you sign up for something midway through a billing cycle, but obviously that does not apply here. As a customer we are responsible for our billing. If we do not want to be charged a full month for a partial service, we have every opportunity to cancel our subscription before that billing cycle. In other words, its not really a billing problem at all...unless they never tell you its coming. But we know its coming, so its a non-issue.