Unfortunately, there is no way to lower the interval time from when you reset it. However, these cars have "oil condition sensors" which allow the service interval to actually set itself. By sensing the condition of the oil, the 15,000 mile service is no longer an exact mileage due to the way different people drive. Therfore, with it only being about 3000 miles difference, i dont think you should be worried, if you had only 1,000 miles left, then i would be a little worried, but that shouldnt be a problem. So, you dont have to be worried, for the fact that it will still tell you when it's time to bring it in for service.
I ahve done services on vehicles with 10,000 miles, and some with 20,000 miles, nowadays its just how people drive.