There's no way to make money from selling code for converting dates, even if that code converts between dates in different calendar systems. To be used one has to supply either the source code or a DLL (or similar), and as soon as it's been released it's likely to be widely available on the net (for use at no charge). The only way such code can repay the time and effort spent in development is by being incorporated in a date conversion application -- see examples [here](https://www.hermetic.ch/astronomy-calendars-date-conversion.php) -- but I suppose there's very little demand for such software.