Donna: I grew up.
Tanya: Well, grow back down then!
Harry Bright: Is your father here?
Sophie: I don't know. I don't know who my father is.
Harry Bright: Bugger.
Sam Carmichael: My sentiments exactly.
Donna: What are you doing here?
Bill: I'm writing a travel piece.
Harry Bright: I took a spontaneous holiday.
Sam Carmichael: I thought I'd drop in and say hi.
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.