Same thing happened to me and my 09 coupe last week. not only was the clock randomly get reset or goes 5 minutes or so slow per day, the things you will also observe happening include no more welcome lights, and no residual heat functionality after you turn off the car. Took it to the dealership and the battery was replaced under warranty.
The indy have good reason to get you to go to the dealers for this one. as the car will treat (use and charge) the battery based on it's condition and deterioration. So right now your car is using and charging the almost dead battery as an almost dead battery. if you just stick in a new battery, even if it's a BMW OEM battery, without having the dealership register it, the car will continue to think and treat the new battery as an "almost dead" battery. basically it's going to kill that new battery you put in pretty quickly. Registering it will let the car know to use and treat the battery as a new one.