OFFICIAL Pokémon Timeline by Toshinobu Matsumiya

FatherofGray says: In case you are unaware, Toshinobu Matsumiya has been a key member of Game Freak's staff since the dawn of Ruby and Sapphire. He has participated in many roles with the Pokémon games since then, but has been the game scenario director in all of them. When asked how long the time gap between G/S/C/HG/SS and D/P/Pt to B/W is, he simply replied "Elapsed time is a secret. Imagine freely!"