Visible crew/equipment: When Donna is telling Tanya about Sophie's dad arriving, stage lights can be seen in Donna's glass.
Visible crew/equipment: When singing Dancing Queen an extra is seen falling off a ladder. If you look at the front of his shirt, a safety cable can be seen. (00:37:15)
Visible crew/equipment: While Sophie's with Lisa and Ali at the dock, after they admire Sophie's engagement ring Lisa bends down to pick up her handbag, and there's a rather large bulge at her lower back under her top - presumably the mic pack, which wasn't there when Lisa lifted Sophie and spun her around.
Visible crew/equipment: Near the end, when Rosie is asking Bill to Take A Chance On Me, she falls and is hanging from the roof. In one shot when she has her glasses, a light screen can be seen reflected in them.
Visible crew/equipment: When Sam is proposing to Donna, you can see a screen reflected in Rosie's glasses.
Visible crew/equipment: When Donna's friends are singing to Dancing Queen you can see a crew member reflected on the hairdryer.
Visible crew/equipment: At the end of a song near the end at the wedding reception, the stage lights and a light screen can be seen in the champagne glasses of the guests.
Visible crew/equipment: When Donna and her friends are singing "Super Trooper" at the bachelor party, you can see a crew member reflected on the colorful door.
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.