One locale which uses ISO 8601 timestamps is en_DK.
I'm guessing KDE 5 returned to the POSIX locale mechanism for consistency. KDE 4 was the odd one – practically all other programs use the POSIX-format locale settings, with predefined formats. (This means the settings will be understood by non-KDE programs, too.)
If KDE doesn't list en_DK, grep the output of
locale -a to check whether the
en_DK.utf8 item is available. If it isn't, on Debian/Ubuntu you should be able to add it via
dpkg-reconfigure locales. On other distributions, if there is
/etc/locale.gen, add (or uncomment) the following lines to it:
locale-gen to rebuild.
If KDE still doesn't list en_DK despite
locale -a showing it, you should still be able to set it globally. The above formats correspond directly to POSIX locale environment variables:
- Region →
- Numbers →
- Time →
- Currency →
- Units →
- Sorting →
The system-wide location for these varies. Often you can set them via
localectl set-locale LANG="en_US.UTF-8" LC_TIME="en_DK.UTF-8"
Sometimes you'll have to edit
/etc/default/locale, or similar.
Per-user, the same can be set in
~/.bash_profile, or similar.