Related anecdote: Interestingly enough, Excel doesn't support dates prior to the "Excel epoch". I had some genealogical spreadsheets that I had to move to Google Docs from Excel because Docs supports arbitrary years and Excel treats anything prior than 1900 as strings instead of dates, losing the ability to perform date/time operations.
Sure, but I'm mainly talking about using a spreadsheet just as a spreadsheet. If you enter a bunch of mixed pre-/post-1900 dates into a column and then change the formatting to display as "DD-Mon-YYYY" everything will change except the pre- dates. There isn't really anything complicated needed to make that work, implementation-wise.
