16 CFR Part 425: "Rule Concerning the Use of Prenotification Negative Option Plans"
These companies that automatically charge your account if you don't cancel within so many days usually start the first day on the day they send it, and so you don't even technically get the full trial period because it is about a week in the mail to you. This is a very harmful practice, and shoppers should not have to worry about canceling the product by such and such a date. The sellers should be the ones who have to get your permission to extend the offer at the end of the trial period. That would protect the consumer.