I installed KaOS Plasma 6 using en_US defaults. This is reflected in the dates I can see in Dolphin. However, in the Konsole, I see European dates, even though my locale seems to be set to en_US:

 mparillo  ~  KaOS-Notes  locale                                                                                                                                                                                              1   main 
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC=en_US.UTF-8
LC_TIME=en_US.UTF-8
LC_COLLATE="en_US.UTF-8"
LC_MONETARY=en_US.UTF-8
LC_MESSAGES="en_US.UTF-8"
LC_PAPER=en_US.UTF-8
LC_NAME=en_US.UTF-8
LC_ADDRESS=en_US.UTF-8
LC_TELEPHONE=en_US.UTF-8
LC_MEASUREMENT=en_US.UTF-8
LC_IDENTIFICATION=en_US.UTF-8
LC_ALL=
 mparillo  ~  KaOS-Notes  ls -l                                                                                                                                                                                                    main 
total 160
-rw------- 1 mparillo mparillo 139156 04.10.2023 20:32 octopi-sysinfo-287688756.log
-rw------- 1 mparillo mparillo  17119 04.10.2023 20:32 pacman.log-2023-10-04
-rw-r--r-- 1 mparillo mparillo     12 24.09.2023 09:18 README.md

Is there another setting I missed?

If you installed with an internet connection present, Calamares defaults to using dates & number formats according to the IP address/location found. You can override those defaults in the Locale page (page with a map), lower right hand side.

I am pretty sure I had the Internet working and I did not override the default (Eastern US). In addition to my memory, the Region & Language all show correctly in KDE system settings as well as the local time in my clock plasmoid. Plus the output of locale above looks correct to me. The timezone and date output here:

 mparillo  ~  date
Thu Oct  5 07:35:48 PM EDT 2023

Also seems to reflect my locale. But not the output of ls -l.

It looks like this is it:

 mparillo  ~  grep time .bashrc
alias ls='ls --group-directories-first --time-style=+"%d.%m.%Y %H:%M" --color=auto -F'
alias ll='ls -l --group-directories-first --time-style=+"%d.%m.%Y %H:%M" --color=auto -F'
alias la='ls -la --group-directories-first --time-style=+"%d.%m.%Y %H:%M" --color=auto -F'

To restore my locale, all I needed to do was comment out the line:

 mparillo  ~  grep time .bashrc
# alias ls='ls --group-directories-first --time-style=+"%d.%m.%Y %H:%M" --color=auto -F'
alias ll='ls -l --group-directories-first --time-style=+"%d.%m.%Y %H:%M" --color=auto -F'
alias la='ls -la --group-directories-first --time-style=+"%d.%m.%Y %H:%M" --color=auto -F'

But that gives me an idea that I could use your model to generate ISO-formatted dates.
Do you know what generated my .bashrc?
Was it in Calamares?
Possible candidates?