The Simpsons is mostly one timeline, however, it's all set in one really long year and there's multiple January - December time passes within this single year. Also, because of a sliding timeline, you'd have to adjust flashbacks and flashforwards regarding characters with Year -1, Year -2, etc. That, or you start the timeline in the year the first season came out, have the repeating years occur in real time (as in, you acknowledge that they take place in the years they released but denote them all as Year 1: 1989, Year 1: 1990, Year 1: 1991, etc.), the flashbacks are deducted back from 1989 ignoring whatever modern reference they have, and the flashforwards will always be Year 2, Year 3, etc., until the show finally ends and you can asign real years to it.
Oh... and somehow you have to fit in Futurama and Family Guy stuff, so really, it's a fucked timeline.