The ancient Romans called the days of the month, referring to the Calends, Ides and Nones, and that is what this application does: by entering a date, shows his wording in Latin (one of several). For ancient dates it shows some information about the organization of the pertinent calendar. The application considers the Gregorian and Julian calendars and a reconstruction of Numa and Romulus (or Roman) calendars.
There is also a converter that uses the latin numbers subtractive principle (4 = IV) and, for numbers beyond 3999, the convention: a line above = per one thousand, two lines above = per one million. They can be inserted Arabic or Latin digits.