This is actually pretty common. The pre-post-date is the shelf availability date of the magazine.

What really annoys me is when I buy a book and the copyright date is listed as the next year. I don't understand how that can possibly be legal.

Copyright is automatic and enforced when you register it. Having the (C) notice is actually optional once you register it.

That's a sneaky trick with plenty of scope for abuse. If I postdate the copyright on a book by five years then, after 20 years or so, it will be harder to determine the true copyright expiration date.

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.

That's not how things work.

Well, how does it work if people postdate copyright?

In the US, copyrights are automatic. You can register pieces of work as a way to prove that the work was created at a certain date and time so that you can sue somebody in civil court for damages but the central office is not the mediator for copyright disputes.

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.