When Happy Days came out it took us back about 20 years. That would take us back to the 1990’s today.
Heck. Happy Days debuted over 40 years ago.
Even by the time Happy Days hit 50s nostalgia was already an institution. With Shanana and Grease and American Graffiti it was already going strong. I don’t get a decade as popular for entertainment as the 50s.