The pokemon game's main series scenario writer, Toshinobu Matsumiya, has posted on his twitter 12hrs ago regarding the question about the series' timeline. I know most of us probably has already figured out this by now, but it's still nice to have an official confirmation from the staff


So basically
RBY/RSE
3 years later
GSC/DPPT
few years gap
BW
2 years later
BW2/XY

here's the original tweet
https://twitter.com/matsumiyan/status/464052839870787584