I had a book from my WL come up on 12/9 and I promptly accepted it.  It was accepted by the sender on 12/10, and marked mailed on 12/11.  I waited and waited, and finally PBS marked it lost on 1/6.  Like magic, it shows up in my mailbox today, with a post mark of 1/7.  I've read that sometimes the PO doesn't postmark it right away in it's route, but a month later?  I have a feeling that the sender forgot to actually send the book.  Should I say anything, or just be glad I have the book?  Luckily it was a WL book, or I might have ordered another copy in the meantime.  I'm not mad, just a bit suspicious.  ;)

The most accurate indicator Laura, is counter postage.  It's always dated correctly.  Postmarks are not usually off by a month, but it's difficult to tell.  If the package does not have counter postage, just note the postmark date in the proper place when you log the book in.  You may also wish to add an additional comment, and clarify whatever is readable on the outer wrapper...JIC TPTB have cause to review the senders account, they will have the most accurate information you can provide.

What Denise said.  When you put the postmark date in the mini-survey section when you mark the book received, it will prompt an auto-email from PBS to the sender reminding them that late mailings are not acceptable.

I will do that - thanks!

Also if it is postmarked verify if the postmark matches the sender's zip code or if it is a different zip code. Most items get postmarked either by the sender's home office or the receiver's home office, it's unusual to get a postmark in-between. If it was postmarked at their PO it was mailed late. If it was postmarked at your PO, it's impossible to tell.

A postmark  indicates the date and time that the item was delivered into the care of the postal service.  A package would never be postmarked at the end/receving postoffice. 

The only time it would be postmarked differently, or might have an additional postmark would be if damage & had to be repacked by USPS.

A digital postmark should be no more than 24 hours from actual drop at USPS.