Many credit card issuers offer introductory rates that last a certain number of billing cycles rather than months. The result is actually a shorter introductory period since billing cycles are usually shorter than a full month. For example, an introductory rate lasting 12 billing cycles would actually be around 10 months, assuming a 25-day billing cycle. An 18-billing cycle introductory rate would be around 15 months. It's important to keep track of the billing cycles as they pass so you know when your introductory rate will expire.