Is it a Detective Novel or Historical Fiction?

I think of LIFTERS as an historical novel, though I don't think that's where it would be shelved in the bookstore.  I'm not a big fan of the Maisie Dobbs novels, but the category is similar: a female detective working in a past period of history, post-WW I in Winspeare's stories.  World War II must be about the cut-off point.  If you wrote a detective story set in the sixties or eighties, you'd be careful to have the right president in office and no cell phones, but your novel wouldn't be called historical fiction.  Maybe that's because, though the slang will be different, your characters talk and act the way people do now.  Avoiding anachronisms is a different thing from creating a past milieu.  

In my next post I want to write about the challenge of immersing yourself so far into a past period you forget the reason you've immersed yourself is to write a novel.