Well,like GLH says,better to be early than late for dinner.
If you don't want to be out at midnight,messin' with a cow,and you think you want a little more smoke,you can always start it around 8 PM,down around 190*-200*.
Kick the temp up,when you arise in the morning.
Depending on what and how you are cooking,a brisket could hold several hours in a warm cooler.
You don't mention what he sold you, as to size,grade,etc.,and whay you want the brisket to end up as.
If you'll put your location in your profile,we can usually guess the answers to some of these questions.
These could drastically affect you timing.