The following contains spoilers for Spider-Man: Homecoming
Nearly everybody who has seen Spider-Man: Homecoming would seem to agree that the new movie is pretty good. It adds a great deal to the existing Marvel Cinematic Universe. However, one thing it seems to have added has many fans more than a little upset. It appears to have added a major conflict in the existing MCU timeline. Dates no longer seem to add up based on the new information in Homecoming. Let's break it down.
The major issue comes right at the beginning of the new film. Our opening scene depicts Michael Keaton's Adrian Toomes as he leads a work crew in the dismantling of the debris left behind following the invasion of New York in the original Avengers film. Work has been going on for some time, as Keaton already has an idea as to how to deal with some of the alien artifacts. However, it's still early enough that Toomes is left in a very bad way when he's told he's being kicked off the job to make way for Stark Industries. We're then told that when we zip forward to the main story that it's taking place eight years later. That's where this all starts to fall apart.
Our previous understanding was that The Avengers takes place in 2012, which if true, would place the events of Spider-Man: Homecoming in 2020. There's nothing inherently wrong with this, except that we also know that Homecoming takes place more or less immediately following the events of Captain America: Civil War. This would mean that Civil War must also take place in 2020. Only that doesn't work either. In Civil War, Vision makes reference to it being eight years since Tony Stark revealed himself to the world as Iron Man, which means as of Civil War, we're only eight years into the entire MCU. Even if we start shifting things around in the timeline to fit as best we can, this would still mean that both Iron Man and the Avengers took place in about the same year, and that doesn't really add up.
Also, just by the way, Spider-Man: Homecoming doesn't take place in 2020. It takes place in 2017. There are a couple of pieces of information that would seem to confirm this. At one point, Spider-Man's on board AI "Karen" informs him that Aaron Davis, the character played by Donald Glover, is 33 years old, with a birth date of April 1984, which means it's 2017. We also see the date of the high school academic decathlon as taking place in 2017, as well.
So now, we take that information and work backward and we have to say that The Avengers happened not in 2012 but in 2009 or so. This works with the Civil War/Iron Man connection, as Iron Man is supposed to take place in about 2010, but that still puts The Avengers right on top of it.
And that doesn't actually work either, because the Battle of New York has a fixed date in the history of the MCU, May 4, 2012 (the actual release date of The Avengers in the USA). The New York Bulletin newspaper seen in Daredevil which reports on the event is dated the day after, May 5. So that movie happened in 2012, we can't get around that. Spider-Man: Homecoming is apparently simultaneously taking place in 2017 and 2020. Welcome to Shroedinger's movie. It's not a cat, it's a spider.
Any way you slice it, Marvel, which up to this point has been very strong in their continuity game, appears to be breaking down. It was bound to happen sooner or later. Eventually, it becomes incredibly complicated to keep all this straight, which is why the comic books all these movies are based on have had to reboot themselves now and again, sooner or later, things don't fit and a giant mess gets made.
The whole thing seems odd as obviously nobody threw together an "8 years later" title card on the fly. Somebody figured this was the right time period when even this cursory glance shows a major conflict. In the end, the exact timeline is hardly important to the vast majority of movie audiences, but for continuity-obsessed fans, this is a problem, and there's likely no easy way to fix it.