This page shows how the shortcodes for the Roman Numerals WordPress plugin works:
will convert 2013 into Roman numerals, as follows: MMXIII
will convert MMXIII into a number, as follows: 2013
For dates, the shortcode XXVI/III/MMXV will return the Roman numeral version of the date in the format you request (i.e. UK, US, YMD, Y only). In this case we are using UK format: XXXI/XII/MMXIII
If no date is provided, it will return today’s date, e.g.: XXVI/III/MMXV will return XXVI/III/MMXV today.
You can also use the widget to right in your sidebar, and there is an object-based tool available for more experienced developers.