As previously mentioned, date formats in Ventura and Sonoma have become
less flexible.
Some asshole at Apple decided to straightjacket people into regional
"settings" and stuck with that.
The panel in settings to customize dates is gone
eg: from Mojave:
https://www.dropbox.com/scl/fi/77wsfpibpop83ulewcbri/MojDates.png?rlkey=82cn1invx6awk8iiglz5kj4zw
Digging around, I've found
https://discussions.apple.com/thread/254773234
So the terminal commands:
alanbrowne@Alans-iMac ~ % defaults write NSGlobalDomain
AppleICUDateFormatStrings -dict-add "1" "y-MM-dd"
alanbrowne@Alans-iMac ~ % defaults write NSGlobalDomain
AppleICUDateFormatStrings -dict-add "3" "y-MM-dd"
alanbrowne@Alans-iMac ~ % defaults write NSGlobalDomain
AppleICUDateFormatStrings -dict-add "2" "y-MM-dd"
alanbrowne@Alans-iMac ~ % defaults write NSGlobalDomain
AppleICUDateFormatStrings -dict-add "4" "y-MM-dd"
alanbrowne@Alans-iMac ~ % defaults write NSGlobalDomain
AppleICUDateFormatStrings -dict-add "5" "y-MM-dd"
The above is over-kill, and I'll refine it over time - I think only
option 5 is needed.
Sort of fixes the problem - Finder, as long as the date field(s) are
narrow enough (but not too narrow), now display date-time as I want it.
Now to see how apps behave as other reports about Excel treatment of
date/time on Ventura+ have issues. I haven't installed Office yet on
the M3 iMac.