I believe that these days, years would be written almost all of the time using Western/Arabic numerals in the Gregorian calendar.
But they must sometimes be written using the Hanzi numerals, e.g. in historical or traditional contexts, at least before Western influence arrived.
When it is done, is it done just using the basic digits 一二三四五六七八九十 or does it also use the characters for thousands and hundreds, etc: 千, 百, etc? And is a zero character used, and if so, is it 〇 or 零?
And is there any traditional alternative non-Gregorian calendar which is or was used, in the mainland or in Taiwan?
Are the "financial" / "uppercase" digits 壹贰叁肆伍陆柒捌玖 ever used for writing years?
(I'm making an app for language enthusiasts and language learners and want to include as many variants that make sense but not any absurd combinations that have never been used.)