Have ISO date in KDE lock screen

Do:

sudo vim /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/components/Clock.qml

Find the line:

Qt.formatTime(timeSource.data["Local"]["DateTime"])

Change it to:

Qt.formatTime(timeSource.data["Local"]["DateTime"], "hh:mm:ss")

This changes the time format.
Then find the line.

Qt.formatDate(timeSource.data["Local"]["DateTime"], Qt.DefaultLocaleLongDate);

Change it to:

Qt.formatDate(timeSource.data["Local"]["DateTime"], "yyyy-MM-dd");

This changes the date format.

This way your sistem presents date in the format YYYY-DD-MM, just like God, ISO and Gary Houston intended