Synchronize your watches. The future's coming back...
Why it is hard sci-fi
The film commits to two ideas most time-travel films dodge. First, a timeline can be split into a divergent branch by a single causal change, and once you re-enter from upstream you arrive in the branch. Second, an object carried back in time can recursively redirect history. Biffs sports almanac is the experiment. The rules hold.
Science inside it
Branching timelines as a working model. The film also does the obligatory paradox dance, where Marty in 1955 from the first film is still happening while Marty from 2015 enters the same year. The two Martys are kept on disjoint paths so the rules survive.