When I worked in the travel industry, our computer reservations system (CRS) was SABRE and we could quickly perform mathematical computations on dates with the system. For example I need to know what day it would be 100 days prior to 11/14/2009.
I've yet to find a readily available calculator or other system that lets me perform those same functions.
I did discover that I can get something similar to those answers from the www.timeanddate.com website as one of their tools.
As you see from the screen grab above, all you have to do is put in two dates and it will tell you how many days passed between those two dates.
The www.timeanddate.com website has plenty of other great tools. A little more uncovering and I did find the date calculator that gave me exactly what I was looking for earlier. It would still be nice to be able to do these calculations with a calculator, but oh well.
As an alternative, you can also check-out www.thetimenow.com which is formatted better for the sight impaired and is fully WCAG 2.0 compatible.