The time period of your story is not a genre but a setting that can fit with virtually
any genre. A period piece is a story set in the days of yesterday (a story
set in the future is considered fantasy or science fiction). Road to Perdition, for
example, is film noir but also a period piece. The Green Mile (in the fantasy,
horror, and suspense genres) is a period piece, too, as is Catch Me If You Can.
Unless you’re doing a studio-financed picture with a healthy budget, avoid
period pieces, period. They require special art direction, wardrobe, and props
dealing with that specific time period.