Anyone with an IQ above room temperature knows Z-A is set in the past, but when, exactly? Arceus seems to be set around 1888, given Team Galactic members talk about their work on the Galaxy Hall, based on the iconic former goverment office of Hokkaido, the Red Brick Office. This would also fit nicely with Z-A if we see the Prism Tower as the likely final goal of Lumiose City's restoration, since it's based on Paris' Eiffel Tower, completed in 1889.
However, there's another point in the timeline where Japan and France's history could meet. The Red Brick Office suffered a big restoration after a fire and wouldn't be recompleted until 1911, and only 16 years later, the famous Haussmann's restoration plans for Paris would truly come to an end in 1927. If we consider Poké Balls were said to be created after 1925 in the old Pocket Monsters Encyclopedia, both Arceus and Z-A could fit perfectly in 1926-1927, just noting that in this case, Team Galactic's work on the Galaxy Hall would be a 14-year-old restoration work, not something new they built just a few years ago.

Still, we have to consider not all material in the Pocket Monsters Encyclopedia is still canon nowadays, and that Z-A and Arceus could take place in different eras, really (ie Arceus could occur in 1888 and Z-A in 1927).
What do you think?