You can work around the library lending issue a few ways. Even if the reader has a clock/calendar, you can manually change it.
It is true that you can't use calibre to convert books from the library to read on a Kindle because of the Digital Rights Management. Some libraries do offer mobi format books which the Kindle can read, though.
If you are down to these two choices it's probably best to go with the nook, since it reads epub format (the most common ebook format, and the closest to a "standard" format there is so far) natively and the Kindle is the only reader on the market that doesn't.