If we're talking watching order, First Class should be watched right before Days of a Future Past, because it doesn't add anything to the first trilogy and Origins, but it's extremely important for the new films.
If the timeline had changed by 2023 then this doesn't explain anything, much less the ages of the characters or things like Charles and Magneto building Cerebro
I still think that there are two timelines before Days of a Future Past. One where it lead to X-Men Origins, and the X-Men Trilogy, where we can place all those references like Charles and Magneto building Cerebro, or Mystique not having interactions with Charles or Beast like they would if the...
I mean, the book references the Guardians after the Ms. Marvel pages, so it probably will put all of the film (except a bit with Jane and Darcy) after Ms. Marvel
For now it seems that this will be the timeline in the book:
The Distant Past: (pg. 10-27)
-Eternals (13-21)
-Thor: The Dark World (17)
-Thor (24)
-Shang-Chi and the Legend of the Ten Rings (25)
1940s, 50s, and 60s: (pg. 28-43)
-Captain America: The First Avenger (30-37)
-Agent Carter (38-39)...