How to set system Short Date to ISO format "yyyy-MM-dd" in High Sierra?
I think I found the answer myself (at least I have what I want):

My problem:


System Preferences... > Language &amp; Region > Advanced... > Dates

What I expected was the option to open the Day of Month drop down and select an option with a trailing zero, just as the digit Month allows:



But I don't see that:



So when my Googling came up empty, I coincidentally copied the contents of the Short field 



into a text editor and this is what I found:



So I got the bright idea of pasting "yyyy-MM-dd" into the Short date field and seeing what happens. Sure enough, worked like a charm:



This also worked with getting the trailing zeros in Excel on the year:

Before:



After: