Details on exact expiration datetime of an SSL certificate?

Almost all cert vendors will renew a cert for the additional whole year (or whatever time frame) for a month or so before the previous expires. So if your cert was good for Dec 10, 2010 to Dec 10, 2011; you can get a new cert in November and it'll be good for Nov 20, 2011 to Dec 10, 2012. That way you don't have to worry about "getting the most value out of it".

To answer the question, certs specify the time down to the minute, and include a time zone.

You can feed your public cert through openssl x509 -in Certificate_File.pem -text and it will output the Validity range. The following is from my personal websites from last year:

Not Before: Apr 20 20:48:59 2010 GMT
Not After : Jun  5 01:52:13 2011 GMT