Board Thread:General Discussion/@comment-28986801-20170212232334/@comment-679780-20170213234725

I too think the games take place roughly the present time of their release... except Sonic Mania which takes place during the 90s but thats an ellaborate theory of mine.

And I do not know if this counts as legit but Sonic Adventure did had DLCs for Halloween, Christmas and Y2K. While perhaps these DLCs are not canon, I still overall agree with 123 that the games take place in roughly the present time of their release.