Only fifteen years in the past!
Something I find kind of wild to think about is that although the movie is a period piece set in two different eras, the 1959 storyline occurred only 15 years before the movie came out. That would be like a movie right now set in 2009. But although I am too young to remember 1974, it does seem like that particular 15 years must have seemed longer to people at the time. So much cultural change had happened over the interim, not to mention a big bloody and divisive war that the US had entered and lost during that span, the whole civil rights era, several different presidential administrations and assassinations plus Watergate, and for that matter the scenes in Cuba were from just before Fidel Castro had overthrown the Batista government.
And the cars, clothing, and hairstyles changed a huge amount for the average person, especially for young people, from 1959 to 1974. It all kind of boggles the mind, and it had to be very disorienting for people who were already middle-aged in 1959 and were just a little later into middle age by 1974 and found themselves in a world that had so profoundly changed. Which also explains why you had such popular nostalgic shows and movies like "Happy Days", "Laverne & Shirley", and "Grease", despite it being the same relatively short time span since the period depicted. I don't think in any other time in modern history there has been a nostalgia boom that occurred less than 20 or 30 years later, but someone can correct me if I'm wrong.