Yeah, there's bunches. I know that some Thai's use a funky calendar, so I did a search on Google for "calendar thai gregorian," followed a link to
Cyndi's List - Calendars & Dates, and finally found
Cumberland Family Software's Universal Calendar Converter.
Three caveats:
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Windows-only (written in Clarion, FWIW)
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It's a great example of how NOT to design a user interface
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They're charging $25 bucks for it, and two other programs
When I get some gumption (I'm fresh out), I'll write a calendar converter in perl (or whatnot).
C²
Oh, yeah, I was disappointed that they didn't have Aztec, Mayan, or Incan dates. That's why I'll write one, if for no other reason.