The dates are real, the events are not - except when fictional characters participate in real events, then the events ARE real, sort of, unless they're different, in which case they're sort of real, but not really.
Seinfeld1989TV Series | Source Timeline Roger McDowell spits on Kramer and Newman after the Mets lose to the Phillies when Keith Hernandez makes an error. Kramer and Newman initially thought it was Keith Hernandez, but it was Roger McDowell. Newman: "Nice game, pretty boy." (Season 3 episode The Boyfriend)
Seinfeld1989TV Series | Source Timeline Date of photograph in Christie's apartment that Jerry spots, in which she's wearing the same black and white dress that she's always wearing every time Jerry sees her. (Season 7 episode The Seven)
Seinfeld1989TV Series | Source Timeline Date of Kramer's and Newman's Kramennium/Newmannium party. it's two and a half years from now, but Kramer's already storing chairs in Jerry's apartment. Newman started planning back in 1978, and plans to feature Christopher Cross. (Season 8 episode The Millennium)