you said it was 15 lb SO... At 275 deg without wrapping figure 1 hr lb after trimming + 2 hrs rest time. Now you have corrected yourself saying it is 13.6 lb in the package. Average trim is about 2 lbs so go long and figure it will be 12 lb. = 12 hrs Cook + 2 hrs rest 14 hrs, have it on by midnight you should be good.
I was not including foiling (I don't foil brisket) in my 1.5. That was including rest time though. The OP made it sound like it had to be done at a specific time, so I'd rather be safe. That being said, foiling does make the time to finish safer. I might go that route and shorten the cook with a hard deadline.