This is actually pretty common. The pre-post-date is the shelf availability date of the magazine.
I'm not a lawyer but I've got a countermeasure for this type of legal abuse. Copyright is intended to be for a fixed term but I don't recall any details that allow or prevent this period to be deferred. Therefore, if a book has next year's copyright then it may not yet be in copyright.
It is that registration date that's important, not so much what the author put down as the copyright year. So people that post date copyrights will have a hard time convincing a court that they truly own the copyrights without any other supporting evidence (such as registration, notarization, or other means of proving a date).
And I'm not aware of anyone post dating copyrights years ahead. That would make no sense as copyrights firstly last decades after the person's death and secondly can easily be extended. So delaying the "start" date of a copyright has no particular advantage.