The pokemon timeline is weird but we know some things like how the fact that BW2 and XY apparently take place in the same year with BW2 happening earlier in the year and XY happening later in the year. We had a tweet from one of the developers of XY, about the timeline…but that has been deleted.
Though this could still be canon and reflected in the game itself with Colress, recently had visited geosenge studying the stones, because he knows about the bond between pokemon and trainer is stronger than any machine could bring out because of his arc that he had in BW2. So he likely after disbanding team plasma, he went to kalos in later months and studied the whole innocent that went down with war 3000 years ago prior to the events XY.
Because of this information, we know that USUM (or the gen 7 games) HAVE to take place 2 years after XY and BW2, since Grimsley, in the concept art of SM art book says that he changed his hairstyle (2 years ago) look at NO. 10 on the concept art.
Not only does that mean that XY takes place 5 years before ZA, but USUM takes place 3 years before ZA as well.
Z-A decided to give us some new lore tidbits, that by the time of this game, the events of SWSH have ended so Leon isn't champion anymore. AND A VERY IMPORTANT PIECE OF DATA, type null from SM, which originally only had 3 of itself and Gladion saved one of them (it's his ace) and he gives the player one type-null in SM, meaning there is one left.
The information regarding it's creation was apparently leaked and now people can replicate it, so that means between the three years USUM and ZA, SWSH happens.
Now here is where we get to our problem…where does ZA correlate with SV? Is it AFTER the events of SV or before?
by LowSolution3084