My experience has been that it could be most anything. Year of first printing, year copywrited, year of that edition and , of course, many tines it's blank.

I have gotten books that said 1934 but when it came it was printed in 1967 , from a 1934 copywrite. If it is very important to you don't trust it.

The "publication date" is supposed to be the year that particular book was published rather than the copyright date.  But as Thomas said, it does vary depending on who created the listing. 

And the date is not one of the four things (title, author, ISBN and book type -- plus large type if that is mentioned) that has to match when we post a book here, so if you are looking for a particular edition of the book, I'd recommend using a Requestor Condition.  Sometimes several editions of a book share the same ISBN.

Also, if you see a book with an ISBN, and the date for the listing is 1923, something is off.  The ISBN system was established in 1972, although some books as early as 1967 have usable numbers dating from an earlier system that was revamped.

The date...tends to run by edition.  So...say in the case of classics.  There's an edition of Uncle Tom's Cabin that has a publication date of 1970.  Under that same ISBN, there have been several reprints and in fact you can buy a brand new copy from your local bookstore.  So by requesting that ISBN at PBS, you might get a book that's over 30 years old, or you might receive one that was purchased new last month...all the same ISBN.

Some books will only have one run per edition/ISBN, and some will have a long period of printing under an ISBN (such as classics).  No way of telling from merely looking at the PBS book info page.  Sometimes you can figure it out by looking at Amazon (old publication date, but still available brand new).

Other examples of this type of printing Lord of the Flies (1999), Medea (1993), Animal Farm (1996), and The Odyssey (1998).  All with multiple printings, multiple covers, and currently available brand new...but the same ISBN (per title).