Shoot down Marty's plan to go back to 2015
From Create Your Own Story
You point out to Marty that you'd only going into the 2015 of the new timeline. You tell Marty that he needs to find out from Biff when he went back to. After a while, you fly the DeLorean to Biff's place as Marty jumps out the roof. The two of you escape. Marty tells you Old Biff went back to 1955 on the day he left back to 1985. You comment on the chances of Biff choosing this date.
