Jake shows up at the house on August 28, 1980. Shortly thereafter you see Willoughby is reading and hands over Carl Sagan's Cosmos to Jake. Cosmos wasn't published by Random House until September 12, 1980.
During the "Canadian" card game, Finn uses the term "hoser" making a reference to Bob & Doug McKenzie's Great White North skits on SCTV. The first Great White North was aired on September 19, 1980, a few weeks after the movie takes place.
When Jake goes to Beverly's dorm room she drinks a can of Dr. Pepper. The can is a modern can with a stay-tab and indented drinking lip rather than the straight-walled, pull-tab can that existed in 1980.
In the penultimate scene when Jake goes back to Beverly's dorm room, "Hand in Hand" by Dire Straits is playing. The movie takes place in August, 1980, but the song was not released until October, 1980.
After Jake and the group drive off from an apartment complex and are passing through a neighborhood, an obscured modern blue Toyota and green Dodge Caravan (within two different houses) are briefly visible.
When Roper and Nesbitt are rifling through Jake's belongings, Nesbitt reveals that a copy of "Remain in Light" by Talking Heads is in his record collection. The date of the scene is Friday, August 29, 1980, but "Remain In Light" would not be released for another 41 days (October 9, 1980).
The story begins on Thursday, August 28, 1980 and follows the characters through the weekend leading up to the first day of classes on Monday. However, Monday, September 1, 1980 was Labor Day and most schools would have been closed on that holiday. Since S.T.U. is a fictional school, it could be said to be an exception, but none of the characters even mention the holiday so it seems like an oversight goof.
At Justin's house, a close-up is shown of a phonograph playing track 2 or 3 of an LP with a dark blue label. The song supposedly being played is "Alternative Ulster" by Stiff Little Fingers, but that album had a reddish-orange label and that song was Track 4 of Side 2. The LP in the close-up was obviously just a prop platter.