I think you're talking about season 4 episode 1, It's a Wonderful Life, where Eric has a dream of what it would be like if he and Donna never dated
Hi! I've watched That 70s Show quite a few times (7 I think) and I've noticed this too! In the episode where Eric goes to his high school reunion, it says class of 1979, but later in the show, the gang graduates in 1978, with the exception of Jackie, who graduated in 1979. However, this doesn't make sense, because in the first season, which is set in 1976, Eric turns 17 years old, which would make him a junior in high school, so they should all be graduating in 1977, except for Jackie. They also say that Donna is a few months older than him. The information I found online says that Eric, Donna, and Fez were all born in 1960, Hyde in late 1959, Kelso in 1958, and Jackie in 1961. Therefore, Eric should be turning 16 in 1976. So, I think some of the information online is incorrect, but also the show had messed up the timeline a lot.