PandoraHearts doesn't actually take place in a specific time period. However, the time period it's based on is the Victorian Era. Keep in mind that it's technically not in the Victorian Era despite being based on it due to PandoraHearts not taking place in the same world we live in.
Regarding the time period during the epilogue of volume twenty-four where Oz and Alice get resurrected, that's probably based off of something like the 1960's or 70's. The Victorian Era began in the 1860's, and since Gilbert waited a hundred years for Oz and Alice, that is probably the time they're supposed to be in. Possibly even later, considering we don't know what part of the Victorian Era PandoraHearts was based on. Notice Oz, Alice and Gilbert are wearing modern clothes at this time.
Again, do take note that PandoraHearts is not actually in any of these time periods, but is merely based on them.