I believe that these days, years would be written almost always using western/Arabic numerals in either the Japanese era or Gregorian calendar.
But it must sometimes be written using the kanji numerals. In historic or traditional contexts. At least before western influence arrived.
So I'm assuming it would mostly be the Japanese era calendar when it is done.
But when it's done is it done using just the basic digits 一二三四五六七八九十 or does it also use the characters for thousands and hundreds 千百 etc?. And is a zero character used, and if so is it 〇 or 零?
Would the "financial" kanji for digits 壱参肆伍陸漆捌玖 ever be used to write years?
And if the Gregorian/western calendar is ever used when writing dates in kanji, like above, which way are the years written?
(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.)