The patent office just posted this on their site.  You can download it to your desktop and try it out.http://www.uspto.gov/patents/law/patent_term_calculator.jsp#heading-1 .  Several years ago, we produced a similar program that works really well.  However, one word of caution. You still need to review the file history and look at the exact language in terminal disclaimers and other quirky things that happened during prosecution.  A simple program will usually work, but if you need to know an exact date, check the file history.