[bwburke94]

The Red Gyarados report was only in DP and not Platinum, but let's assume it's canon.

Kanto (R/G/B/Y/FR/LG) occurs in the year I'll call 0 for the purposes of this discussion.
Hoenn (R/S/E) appears to be placed around year 1 based off Steven's cameo in HGSS.
Johto (G/S/C/HG/SS) is confirmed to be in year 3.
Sinnoh (D/P/Pt) is hinted to be in year 3 based off the Red Gyarados and a few cameos in and from Johto.
Unova 1 (B/W) is somewhere around year 6-8 based off Cynthia and Caitlin's appearances and dialogue.
Unova 2 (B2/W2) is therefore somewhere around year 8-10.