Plasma 5.7 still doesn't allow users to configure date-time (and units), and a few other things that really should be user preferences. ISO 8601 needs to be available in all locals.

This isn't a particularly complex issue. I haven't touched C/C++ in a decade (and hated it when I had to use it for a class, at least most of it), or I would start hacking some derived class of the Qt5 locale, and presumably the configuration GUI widget in KCM.

They could get a 90% solution for the complaints I have seen by adding a single (checkbox) option: use ISO 8601 date/time format (e.g. 2017-02-14 15:22), perhaps with an additional "show seconds in time/date".

There are other time/date conventions used in the US (e.g. 14FEB2017 1522 is common in the military, and a more generic option to use 24h v/ 12h am/pm setting needed, but these can all wait until the ISO standard is supported. All formats other than the ISO format are (somewhat) location dependent, subject to convention, and inherently ambiguous.