This apply to my configuration
KDE Frameworks 5.55.0
Qt 5.9.7 (built against 5.9.7)
kf5-config: 1.0
The xcb windowing system
plasmashell 5.12.8
Leap 15.1
kernel 4.12.14-lp151.28.48-default
Documentations :
After reading this :
https://askubuntu.com/questions/783184/how-to-display-kde-lock-screen-time-in-24-hour-format
which cites also :
Wrong time format in plasma5 lock screen
https://forum.kde.org/viewtopic.php?f=289&t=130484
Searching in internet I found this about Qt date time format
https://doc.qt.io/qt-5/qdatetime.html#toString
Current ‘look and feel’ theme : opensuse
Configuration files in : /usr/share/plasma/look-and-feel/org.openSUSE.desktop
As ROOT after having done a backup just in case, edit the current running theme file : /usr/share/plasma/look-and-feel/org.openSUSE.desktop/contents/components/Clock.qml
To change the time from the standard format to hh:mm:ss 24h clock
find a line like :
text: Qt.formatTime(timeSource.data"Local"]"DateTime"])
change to :
text: Qt.formatTime(timeSource.data"Local"]"DateTime"], "hh:mm:ss")
To change the date from the default locale long date format to yyyy/mm/dd
find a line like :
text: Qt.formatDate(timeSource.data"Local"]"DateTime"], Qt.DefaultLocaleLongDate);
change to :
text: Qt.formatDate(timeSource.data"Local"]"DateTime"], "yyyy/MM/dd");
Hit Ctrl+Alt+L to lock your screen and see the change immediately.
This should be applied on each other theme in the folder /usr/share/plasma/look-and-feel if wanted.
I hope it helps someone.