LogFAQs > #976251443

Foppe posted...

It is not that hard to imaginate.
It got three dates, when you are, where you came from and where to go. Most likely he first went back to 85 (Doc most likely input it for the return trip) and he figured out the meaning behind the numbers.
He took a note of the date where he came from, went back to 55, then input the date where he originated from.

Doc makes it explicitly clear that you can't go back to the nice future because Marty's first suggestion was to stop old Biff from getting the almanac in the first place. But how did Biff get back to it then?