I've been hunting for something like this for a long while, so thought I'd share my fortunate find here.

DateDiff is a desktop calendar program that allows you to select two different dates, then calculates the number of days, hours, minutes and seconds between them. It can also determines what the date is X number of days before or after a given date.

318KB download, <img src=/S/free.gif border=0 alt=free width=30 height=15>ware of course!