Adam sees E.T. in the theatre twice. It was playing in theatres in 1982. Later the Beach Boys song, Kokomo, is mentioned by Barry. That song was released in 1988.
Calabasas asks "Does anyone know a four letter word for a Mongolian tent made of skins?" His crossword is probably referring to "yurt". However, yurts are Russian. A Mongolian dwelling is a ger, and is made from felt, not skins.
Adam watches E.T. released 1982 and has a model of ECTO1 from the real Ghostbusters released in 1991.
The film E.T. was in theaters in 1982 but throughout the show Barry is wearing a Beastie Boys t-shirt (with "Fight for Your Right to Party" written on the back). The Beastie Boys released "License to Ill" (which contains the song "Fight for Your Right to Party") on November 15, 1986.