Other mistake: When Harry Bright and Sam Carmichael are standing on the dock in Greece after missing the boat, Harry pulls out his invitation and references the wedding in the invitation, but the envelope is sealed. (00:08:20)
Other mistake: Donna sings The Winner Takes It All and it is nowhere near sunset. During the song she makes her way up to the church. But when she arrives, there is a good sunset.
Other mistake: At the end of them singing "Money, Money, Money", when Donna is screwing the falling window shutter back, there is no drill bit on the drill, making it not possible for her to put the shutter back up.
Other mistake: When Sophie swims to the boat with her three possible fathers on it, when she takes off her clothes to reveal her swimsuit, she takes a very short time to take them off, too fast for them to actually be off.
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.