rbranster is giving you sound advice. Over estimate the time. If its done earlier it will maintain temperature wrapped in towels, if its done late you just get to let everyone starte at the smoker with you.
There is no way you can rush the cooking process, and still get the benefit of cooking low and slow. I don't like the idea of the all-nighter like some do. My long cook I did a few weeks ago, I prepped everything the night before so all I needed was a match in the morning. I got up at 245 am, lit the pre filled chimney, hit the snooze, got up 10 min later, poured the hot coals in, and got up to temp. I put the butt in the smoker, set a temperature alarm on my redi-check and went back to bed until the temp alarm woke me up to stir the coals.
You are gonna lose some sleep on a 15 hour cook no matter what you do.....