How to set ISO date/time with en_US.utf8 as system default?

[gwmfms6]

This wiki is apparently out of date because it does not work: 
https://wiki.debian.org/Locale#First_day_of_week

I have the system default set to en_US.utf8. But I need sensical dates 
and times (and en_US.utf8 uses nonsensical date & time format).

How can I set my Debian 8 stable to use en_DK.utf8 for LC_TIME and 
LC_MEASUREMENT variables while keeping en_US.utf8 the system default for 
everything else?