Lifespan Age Calculator is a quick, small and very effective way to add and subtract dates. Lifespan runs on pretty much anything (anything running windows that is) and is contained in a single zipped file of 35k which makes Lifespan very easy to distribute on the net or by email. This is not a big scale production, just my pet.

You feed Lifespan a starting date and a finishing date and Lifespan will tell you the difference in years, months and days. Very useful for government forms in my country, thats for sure.