I've found the Book by date never holds up -- and the fly by date hasn't either. The fly by dates usually round "up" to the year in which it was to expire....to December 31st.

So I had certs that were to expire September 2011 and actually were valid for use up until 12/31/2011. I have a cert that was to expire 12/21/2011 and its still valid, and I expect it to stay valid until 12/31/2012 since I am able to use the cert on flights well into 2012.