I started this timeline page because by the end of Season 4, the backstory of Bo Jack Horseman and how it relates to the events of the show has become fairly complex. You have the childhood of Beatrice back in the 40's, her relationship with Butterscotch, Bo Jack's childhood and how Secretariat's career affected it, the entire Horsin Around saga, where people were in their lives during those nine years, the many isolated events of 1996 to 2014, and so on.
What I laid out is some of the major events and character birth years, which should help provide context at a glance. I also stuck with reasonably certain dates, with the main exception being Herb being fired in 1993. I feel that year is justified (his departure marked "The Latter Years" of the show, Bo Jack resembles his 1994 self in S 1 E 08, etc), but it may be a bit off.
There are still things that can be added. I did not include the Leo incident from Diane's childhood (1999) because I didn't know if it was worth knowing the context behind it. And we could probably add all sorts of life events for the main characters.
Herb kazzaz is actually fired in 1995, judging by the calendar we see Angela walk past in the episode “Angela”. I made a post about this here: https://www.tumblr.com/bojackandherb/715152818345033728/something-i-noticed-sarah-lynn-who-was-born-in
(I just noticed the previous comment was made in 2017, well before season 6 had aired. It was a reasonable hypothesis at the time but now we know more haha)
I changed the year Bojack drinks the spiked orange juice from 1987 to 1989 because the date of that scene can actually on a clapperboard that flashes during the scene transition. I noted this here https://www.tumblr.com/bojackandherb/715134816688963584/fun-fact-we-actually-know-the-exact-date-the