Pokemon, what happened to you?

[Wes]

Well RSE are supposed to be taking place at the same time as RBGY. That's why you can trade between FRLG and RSE without a Time Machine. Sure you need a special long-distance trade machine, but they take place at the same time. Similarly, GSC take place at the same as DPPt. In Gen 4, you start out the game by watching a news story about a Red Gyarados and your rival wants to check your local lake for rare awesome Pokemon. If Gen 4 took place after Gen 2, the Red Gyarados wouldn't be there. So the timeline sort of goes RBGY/RSE/FRLG ---3 years---- GSC/DPPt/HGSS