So when you have a show that is all about time travel, timelines can be looked at as fluid. Actions in the past, or present, change the future. Something we saw may no longer have validity because, due to the rules of time, something was changed - or at worst, erased.
THIS timeline - the one you see below - is comprised of all the actual events that have occured as of this writing. For instance, an event - or the year of an event - from Season 1 may not be reflected on here because the below is the timeline of the entire series as it exists as of this moment, overridden/changed events included.