One of the most common ways that credit card companies can legally take your money is by playing with the grace period on your charges. Most banks typically allow for a 'grace period' on your account. If you charge something in the middle of your billing cycle, no interest is charged if you pay the bill by the due date. The logic is simple. Now for the newest wrinkles.
Some credit card offers, mostly for 'gold' cards, allow for no grace period.
In other words, if you charge an item on the 15th of the month and bill is due on the 30th, you automatically will be charged 15 days of interest at the stated interest rate. OK, so you threw that gold card offer away. You're still not exempt from billing date issues. Suppose you fail to pay your bill in full one month. Maybe you were on vacation or had a temporary emergency.