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Wolfram|Alpha - the computational engine/database which provides quick answers to mathematical problems and relationships as well as pulls information from its databases of facts - has released a Widget Developer.
This allowed me to create this calendar widget:
The input boxes can be modified from their default values. I was unable to figure out how to populate a pull-down menu, but possible calendars include:
Chinese Lunar Calendar
If you wished to convert from January 1, 2010 - Julian to Gregorian, in the first input box you would enter:
January 1, 2010 Julian Calendar
and in the second input box you would enter
Otherwise, it assumes "January 1, 2010" is Gregorian.
The other calendars have different names for the months, so it knows what you mean.
If you follow the link to the widget above, you can access the code to embed it on any page you want. Or follow instructions on how to customize the widget, or build an entirely new one.