Correction: In the error message, It says there was an error, some or all of this file could not be transferred. (It is already downloaded onto my computer and I can see/read it on my computer). The problem is getting it on to the Nook.
Library ebooks contain information about when they were borrowed and when
they're supposed to be returned. So, it's possible that the time change
you did could mess that part up.
Also, some ebooks from the library could contain digital rights settings
that would not let you transfer them from the computer to an ereader. You
can't change that if it's that way - the library might be able to, but it's
usually something that the publisher set up.
Tell us the exact wording of the error message. Maybe the problem isn't
either of the above!
You must transfer using ADE. When you say "It says there was an error", Is that 'It' ADE?