How is it that GNOME still does not allow changing date format within a locale?

I am using the en_US locale. I need LC_CURRENCY, LC_NUMERIC, AND 
LC_PAPER to be formatted per the en_US locale. But I need LC_TIME and 
LC_MEASUREMENT set to international standards (e.g., en_DK). I can set 
this in Debian, but the GNOME DE overrides it and prevents it within the 
GNOME DE. So why does GNOME not allow the changing of formats within a 
locale? Settings > Region & Language only allows changing the locale. 
Different locales give different formats but do not allow using U.S. 
locale with Denmark's date format (for example). Why not? How has this 
not been fixed yet?

Other Operating Systems (e.g., Mac, Windows) allow me to set Monday as 
the first day of the week and use ISO standardized date formats while 
keeping everything else set to a U.S. locale (something akin to using 
en_US locale with LC_TIME and LC_MEASUREMENT set to en_DK). Why doesn't 
GNOME?