Created attachment 161514 [details]
The Date & Time dialog showing a 12 hour AM/PM format instead of a 24 hour format

SUMMARY
***
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***

STEPS TO REPRODUCE
1. Open System Settings then Regional Settings
2. Set the Language to American English and the Time to the "C" time format near the top of the list. The latter should list the time in a 24 hour.
3. Log in then out again.

OBSERVED RESULT

KDE Plasma is still showing time almost everywhere outside the Digital Clock widget in the Icons-only Task Manager in a 12 hour AM/PM format instead of the designated 24 hour format.

For example, a time of 15:55 is represented as 3:55 PM.

EXPECTED RESULT

KDE Plasma shows the time everywhere in the chosen format. 15:55 should show 15:55, not 3:55 PM.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: EndeavourOS with Linux kernel 5.4.12-arch1-1 (64-bit)
(available in About System)
KDE Plasma Version: 5.27.7
KDE Frameworks Version: 5.109.0
Qt Version: 5.15.10

ADDITIONAL INFORMATION

Output of `env` is:

SHELL=/bin/bash
SESSION_MANAGER=local/user:@/tmp/.ICE-unix/37367,unix/user:/tmp/.ICE-unix/37367
WINDOWID=96469005
QT_SCREEN_SCALE_FACTORS=HDMI-0=1;DP-0=1;DP-1=1;eDP-1-1=1;DP-1-1=1;HDMI-1-1=1;DP-1-2=1;HDMI-1-2=1;
COLORTERM=truecolor
XDG_CONFIG_DIRS=/home/user/.config/kdedefaults:/etc/xdg
FUNCNEST=100
XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session5
LUTRIS_SKIP_INIT=1
FONTCONFIG_PATH=/etc/fonts
LANGUAGE=
LC_ADDRESS=en_US.UTF-8
DOTNET_ROOT=/usr/share/dotnet
LC_NAME=en_US.UTF-8
DOTNET_CLI_TELEMETRY_OPTOUT=1
SHELL_SESSION_ID=14e2a0066d14422a8b3687d9358d71ad
MEMORY_PRESSURE_WRITE=c29tZSAyMDAwMDAgMjAwMDAwMAA=
DESKTOP_SESSION=plasma
LC_MONETARY=en_US.UTF-8
GTK_RC_FILES=/etc/gtk/gtkrc:/home/user/.gtkrc:/home/user/.config/gtkrc
XCURSOR_SIZE=24
EDITOR=vim
GTK_MODULES=canberra-gtk-module
XDG_SEAT=seat0
PWD=/etc
XDG_SESSION_DESKTOP=KDE
LOGNAME=user
XDG_SESSION_TYPE=x11
SYSTEMD_EXEC_PID=37457
XAUTHORITY=/tmp/xauth_oFzxJx
MOTD_SHOWN=pam
GTK2_RC_FILES=/etc/gtk-2.0/gtkrc:/home/user/.gtkrc-2.0:/home/user/.config/gtkrc-2.0
HOME=/home/user
LC_PAPER=en_US.UTF-8
LANG=en_US.UTF-8
XDG_CURRENT_DESKTOP=KDE
KONSOLE_DBUS_SERVICE=:1.338
MEMORY_PRESSURE_WATCH=/sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/session.slice/plasma-plasmashell.service/memory.pressure
KONSOLE_DBUS_SESSION=/Sessions/1
PROFILEHOME=/home/user/Desktop
XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
INVOCATION_ID=579b940f33e84ce28c754edfd4713701
KONSOLE_VERSION=230800
MANAGERPID=1024
KDE_SESSION_UID=1000
XDG_SESSION_CLASS=user
TERM=xterm-256color
LC_IDENTIFICATION=en_US.UTF-8
USER=user
COLORFGBG=15;0
KDE_SESSION_VERSION=5
PAM_KWALLET5_LOGIN=/run/user/1000/kwallet5.socket
DISPLAY=:0
SHLVL=1
LC_TELEPHONE=en_US.UTF-8
LC_MEASUREMENT=en_FI.UTF-8
XDG_VTNR=2
XDG_SESSION_ID=8
XDG_RUNTIME_DIR=/run/user/1000
DEBUGINFOD_URLS=https://debuginfod.archlinux.org
LC_TIME=en_FI.UTF-8
QT_AUTO_SCREEN_SCALE_FACTOR=0
JOURNAL_STREAM=8:122572
XCURSOR_THEME=breeze_cursors
GTK3_MODULES=xapp-gtk3-module
XDG_DATA_DIRS=/var/lib/flatpak/exports/share:/usr/local/share:/usr/share
KDE_FULL_SESSION=true
BROWSER=firefox
PATH=/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
KDE_APPLICATIONS_AS_SCOPE=1
HG=/usr/bin/hg
MAIL=/var/spool/mail/user
LC_NUMERIC=en_US.UTF-8
KONSOLE_DBUS_WINDOW=/Windows/1
_=/usr/bin/env