Just a little advice, since I am going through a major problem with another company with this same type of thing: pay for any month that you end up using, even if it is partially used, as long as you know you will get a credit for unused service. A cell phone company is trying to charge me for an entire month's bill when I canceled 13 days into the 31 day cycle, and I didn't pay the bill when it was due, largely because I had canceled a full week before the bill was even due to begin with.
My statement gets sent out dated the 16th, it's due around the 5th, and goes basically from the 1st to the end of the month for service. Because of my unrelated problem, I actually emailed them a few days ago about this kind of situation, and they said that because they bill in advance, you would pay for the month's service, and then be credited for service that was unused.
Good luck.
Eric