While studying Computer Science at the University of Arkansas at Little Rock, I ran across an algorithm for converting year, month, and day to Julian day number — a serial numbering of days used by astronomers. This eventually led me to research other calendar systems, especially ancient calendars.
RosettaCalendar.com — a calendar converter I wrote many years ago. Someday I will add more calendars to it.
sdncal20.zip — Free, open-source software I wrote in the mid-1990s for date manipulation and calendar conversion. Since it is free, this library has been included in many software packages. Likely the most famous is PHP, the language used by WordPress and over 75% of all websites. If you visit a website that does anything with dates or calendars, it is likely using this library somewhere behind the scenes.