Movie is supposed to be late sixties but....
They didn't say "just say no" until Nancy Reagan said it in the 1980s.
They didn't have break dancing in 1968.
I can't recall exactly when those big two-liter plastic bottles of Dr Pepper first showed up but they didn't exist in 1968.
Julie's dream man (Josh Brolin) seemed to be driving a Cadillac that was made recently. They didn't make Cadillacs that looked like that back then.
Guys didn't refer to other guys as "dude" back in the late 60s.
It's kind of weird that the producers didn't pay closer attention to historical accuracy. It's not really clear when this movie was supposed to be set in but if it was 1968, they really botched a lot of it.