> Edit: Changed this from email because email validation is a can of worms as an example

Email honestly seems much more straightforward than dates... Sweden had a Feb 30 in 1712, and there's all sorts of date ranges that never existed in most countries (e.g. the American colonies skipped September 3-13 in 1752).

Dates are unfortunate in that you can only really parse them reliably with a TZDB.