Actually, I think they start in 1976, because in an early episode (the streaking one) they had President Ford come to town. Ford left office in January 1977 after being defeated by Jimmy Carter in the November 1976 election.
The timeline is really unclear as the series progresses. The characters are not aging in real time. For example, they didn't get out of high school until year 5, I believe. I've also noticed that the topical references from the first season or so are pretty much gone, so I think what's happened is that the producers have frozen the show in a sort of permanent late-70s timeframe. I think they just decided to avoid the 80s thing.
The timeline is really unclear as the series progresses. The characters are not aging in real time. For example, they didn't get out of high school until year 5, I believe. I've also noticed that the topical references from the first season or so are pretty much gone, so I think what's happened is that the producers have frozen the show in a sort of permanent late-70s timeframe. I think they just decided to avoid the 80s thing.