I believe the only way to do what you are asking is to cancel your subscription. Go premium until sometime in the next month. Reactivate.
There is a problem because you do not buy game time in months. You buy game time in 30 day chunks. The date moves back and forth depending on whether it is a 31 day month or it is February which is 28 or 29 days. Over a year the billing date will move backward 5 or 6 days depending on whether it is a 366 day year like 2012 or a 365 day year like 2011.
Unless stated otherwise, all content in this post is My Personal Opinion.