"And my timeline paper beats his, thanks to my superior firepower!"
The chronological timeline of the Apes movies and TV show is quite convoluted, with seeming contradictions scattered throughout. Indeed, many have tried to reconcile the various dates given throughout the series, and their efforts can be found on various sites, such as this one, this one, this one, and this one, among many others (One site that seems to offer an authoritative outline is HassleinBooks.com. I have not bought the book to examine it, but it looks to be the definitive effort so far) . This article from issue #11 of Marvel's 1975 POTA magazine seems to be the first published attempt to track the various dates and events given, in order to try to make sense of it (although it muddies the water somewhat by interjecting the Marvel comics storylines into it). I subscribe to the theory that once Zira and Cornelius come back in time to 1973, they created another timeline that differs from the original, where they did not go back. Their presence (and DNA) cause the ape mutation and uprising to occur much earlier than their history recorded, and I hold that the subsequent film and TV series events are a direct result of this new timeline. In that film the scientific advisor Hasslein thought that the future could be changed, and I think it was, just not in the direction he wanted. (Update: as you can see from the discussion with a knowledgable fan in the comments, there are different views on this.)
The author further submits the possibility, and I tend to agree, that the Zira and Cornelius of the third film may not even be from the same timeline as the first and second film; since it seems so unlikely that they could have located the sunken spaceship, retreived it, repaired it and launched it into space, even with Milo's help. Their technology just wasn't that advanced. The many attempts to catalog the timeline testify to the inconsistencies found in the movies; the carelessness of the writers force us fans into some strange contortions to justify the timeline of the films, indeed! Continuity was not a strong point of the franchise's producers. One thing is certain; we fans have given it much, much more thought than they ever did when creating it. Maybe we should set our brains in "nuetral" and just pop some popcorn and enjoy the films for the light entertainment they were intended to be...
Speaking of timelines, here is one that tracks various events in the real world relative to the production of the Apes franchise and various items associated with it. It's exhaustive, and informative, but ultimately depressing to me, to see how many people that contributed in some way to the Apes story have passed away.