I ordered a book over a week ago.   The member waited several days before accepting the order and then chose the max time to mail.    The Mail By date was Friday 4/15 at 11:00PM.

As of now, (4/17at 1:22PM)  the member still hasn't marked the book mailed - and the book hasn't rolled over to the next in the FIFO line either.    I thought that if the book wasn't mailed a day after the mail by date then the order would roll over.   So I expected it to roll over to the next member sometime in the middle of the night last night.  But that didn't happen.

Is my thinking wrong?   I couldn't find anything in the Help docs about it.

I think it is the night follow the second day.

Yes.  Here from help (search swapping deadlines)

Here is a list of deadlines associated with the PBS swapping process:

• A sender has 5 days to respond to a request.
  • After accepting a request by clicking "I can mail in the next 2 days", a sender has 5 days to mark the book mailed. The deadline is at the end of the fifth day.
  
  
  • After accepting a request by clicking "I can mail later" and choosing an alternate mailing date, a sender has 2 days beyond the alternate mailing date to mark the book mailed. The deadline is at the end of the second day.
• A requestor has 48 hours to respond to an offer of a book from her Wish List.
• A book has 26 days from the "mailed" date to get to its destination within the contiguous US before it is declared "lost in the mail" at PBS.
• A book sent outside the contiguous US (AK, HI, GU, VI, PR, APO/FPO/DPO, etc)  has 35 days from the "mailed' date to get to its destination.

Have they printed the wrapper? If they have, then I don't think it will expire/roll over.

I've had people forget to mark mailed, got the books a week or two later, then had to PM them telling them that I can't mark the book received until they tell the system that they mailed it.

Printing the wrapper won't stop a request from timing out if it isn't marked mailed.  All printing the wrapper does it to prevent the requester from cancelling.

For WL books it times out on the next full hour 2 days after it's offered. So if you post a WL book at 1:29 pm then it rolls over a 2pm 2 days later.  (I think that's how that works).

For book requests it gives 5 days but it's done during the night. It says to mark mailed by 11pm on a certain date.  But it's done later due to time zones.  I thank when the last US time zone hits 11?  So for use the East Coast it won't happen right at 11pm.  Something like that.  It's not on the exact time that you made the request though.

They have 2 full days after the date they select for mailing to mark it mailed, and the system actiivy happens around 3am, so it will cancel tonite at 3am if they don't mark it mailed before then.

Here is the Help Center doc:

You can monitor the progress of your book request from your account page.

To do this:

• Log into PBS
• Click My Account in the main toolbar at the top of the site.
• Click the "Books I've Requested" tab under the Message from the Librarian.
• The status of each request you have submitted appears In the "speech bubble" on the right.
  • "Requested on:" is the date and time that you submitted the request (or that the request was re-submitted by the system if the original request was canceled)
• There are three steps to the sending process (acceptance - printing - mailing).
  • Acceptance
    • If you see "Waiting for Member"
      • this means the sender has not yet agreed to send the book (he or she has 5 days to respond).
    • If you see "Accepted" (with NO "will mail-by" date specified)
      • this means that the sender has agreed to send the book by clicking "I can mail in the next 2 days", and has 5 days from the date of acceptance in which to mark the book mailed.  (The deadline is at the end of the fifth day.)
    • If you see "Accepted" ("will mail-by" date specified)
      • this means that the sender has agreed to send the book by clicking "I can mail later", and has until 2 days after the mail-by date shown to mark the book mailed.  (The deadline is at the end of the second day.)
  • Printing
    • This line will appear after the request has been accepted
    • If you see  "Awaiting Member"
      • this means the sender has not yet accessed the address from the request.  The request can still be canceled by you at this point.
    • If you see  "Printed:" (with a date)
      • this means that the sender has printed your address to send the book (the request can no longer be canceled by you at this point).
      • The date shown here is the LAST time the address was printed.  This will update if the sender reprints the address.
  • Mailing
    • If you see "Awaiting Member" this means the sender has not yet marked the shipment mailed.
      • note that the sender has a 2-day grace period beyond the mail-by date to mark the shipment mailed.
    • When the sender marks the book mailed, the request will move to your En Route to Me tab in My Account, and you will be sent an email from PBS telling you the book is on its way.
    • If the sender does not mark the book mailed by the deadline (2 days after the mailby date), your request will be passed to a NEW sender, and you will see that the "Requested on" date on the request will change.  This will happen without any emails from PBS to you.  (If there are no other copies in the system and the sender does not mark mailed by the deadline, the request will cancel, the book will go onto your Wish List and you will get credit back.  You will get an email from PBS notifying you if this happens.)

Ahhh ... so hopefully it will rollover to the next person tonight!

If I remember correctly from past discussions, non-wl books roll-over to the next person somewhere after 3am EST. So you will probably have to wait until tomorrow to see the new one.  Unless you are a night owl like me.  Then you may be awake to see it roll-over.