Factual error: When you first see Sam he is supposed to be in America, but when you see out of the window as he is leaving, you can see the Gherkin building in London. He is actually in the Lloyd's Building, but to add to the illusion of being in America, he is picked up by a Yellow Cab.
Factual error: Sam and Donna would not be able to marry. There are things which need to be done before it is legal for two people to marry and it is therefore not possible to marry minutes after proposing.
Answer: Sophie finds Donna's diary from 1979, being the year Sophie was conceived meaning she was born in 1980 (as the diary entries are from July/August). So the movie is set in 2000.