At the start of the RP 1850-1860 is the latest. Once the Rp begins you are free to invent all sorts of crazy steampunk nonsense so long as it isn't completely dubious (Flying helicoptor battleships) and so long as it follows a line of progress (No Zeppelins decked out like the Bismark until several smaller protoypes have been developed beforehand).
Uncivilised nations will have to catch up with the modern world first, but otherwise have magic and weird ass creatures to play with.