What year did the Timeskip happen in one piece?

It starts at 517. And it’s only 2 years. Mostly all the characters are significantly stronger and we get some new Admirals and Warlords. The timeskip marks the start of a new “era” where Luffy and the pirates from his generation begin to dominate the Grand Line.

What chapter of One Piece is the Timeskip?

Chapter 598 is titled “2 Years Later”.