the .qml does not parse hh:mm in the date field and there is no toggle for switching things in mine either (don’t think that’s a thing in plasma 5)
you might be able to cut the block of .qml code for parsing the date and paste it above the block that deals with the time but i have no idea if that would break things or not.
run kpackagetool5 --install /usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/
and then you will have a copy of the .qml in your .local to play with… if you break it just delete the directory and start over
You can try out Event Calendar as an alternative clock/calender, this provides you with a freely configurable date/time format string (in the “General” section of the settings). It works very well for my needs - I want to have just the date displayed, and time only by hovering with mouse.