How to display KDE lock screen time in 24-hour format

Mr/Ms user26687 above hit upon the correct answer in his/her comment above, but I can't mark a comment as The One True Answer, so I will summarize the content of the referenced page at kde.org.

As a bonus, that page contains instructions for changing the date format to YYYY-DD-MM, just like God, ISO and Gary Houston intended. Wasn't even asking for that, but got it anyway. Niii-i-i-i-ice.

HOW TO:

1. Go to the directory

   /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/components
   

Therein you will find a file called InfoPane.qml or in 17.04 Clock.qml. Edit it as root.

2. Find this line:

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

   and change it to

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

3. For the ISO date change, find this line:

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

   and change it to

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

4. Save the changes.

5. That's it! You don't even need to log out/in, much less restart anything. Hit Ctrl+Alt+L to lock your screen and see the change immediately.