Technically, you could put both common European digits 0, 1, 2… and Arabic digits ٠, ١, ٢, … as alternative glyphs for the characters U+0030 DIGIT ZERO, U+0031 DIGIT ONE, U+0032 DIGIT TWO, etc.,into the same font, using OpenType features, and you could use CSS tools for selecting between (though this is not yet supported by all browsers). But then you would need to be a font designer, or at least know how to edit a font.
The normal way, however, is to treat European digits and Arabic digits as distinct characters, i.e. make the difference at the character level. So the code that generates the calendar should take care of the issue. And then you just need a font that has both sets of digits, properly assigned to the separate characters.