We're cued to this by the credit sequence and the old optical montage effects. Hanks does not take the artistic license that the intervening 30 years offer him. The PG rating is for language only. There is no sex here. In fact, there are so few kisses that a line about kissing gets the sweetest of the film's many gentle-smart laughs.
Hanks reproduces the period as lovingly as any filmmaker has.
Even the new songs in the movie were written in the manner of the period. It seems significant that Hanks, who wrote several of the songs, has cast his look-alike, Tom Everett Scott, as the story's protagonist, Guy Patterson. Guy works in his dad's appliance store. He has a vague sense that he should do something responsible with his life. But right now, he loves jazz and playing the drums.