I'm a bit surprised that you "have had to program date-related code" and "have never heard of ISO 8601", because it is practically the standard for date formatting. I know non-programmers from different countries including US/UK/EU, and although they probably haven't heard of ISO 8601 either, they have absolutely no trouble understanding YYYY-MM-DD HH:MM:SS. This is a very small sample size and probably not representative, but I doubt many people would attempt to parse it as YYYY-DD-MM.
No country, including the US, uses YYYY-DD-MM:
https://en.wikipedia.org/wiki/Date_format_by_country
I'm a bit surprised that you "have had to program date-related code" and "have never heard of ISO 8601", because it is practically the standard for date formatting. I know non-programmers from different countries including US/UK/EU, and although they probably haven't heard of ISO 8601 either, they have absolutely no trouble understanding YYYY-MM-DD HH:MM:SS. This is a very small sample size and probably not representative, but I doubt many people would attempt to parse it as YYYY-DD-MM.