When is this movie set?

Sky talks about building a website for the hotel, which would imply it's at least the late 1990's. That would make the summer Donna spends with the men no earlier than sometime in the late 1970's. This would corroborate Harry's statement about trading a Johnny Rotten shirt for the guitar, since the Sex Pistols didn't form until 1975.

But during the song, "Our Last Summer," Bill talks about how it was during the age of the Flower Power, which (to me, anyway) refers to the late 1960's/early 1970's. Thoughts?