An actual, working Roman numeral calculator. (Requires a
Java-compatible browser.) It also has explanations of Roman numeral
syntax, conversion, and the Roman calendar.
It's a mouthful, but that's pretty much what's on this site. It's
all written in JavaScript, so you'll have to have a JavaScript-enabled
browser and have JavaScript turned on to use the converters here.
This explains everything you wanted to know about reading years
written in Roman numerals, including alternative forms and the
apostrophus. Much more in depth than anything you'll find here.