24h clock setting not honoured

Henning Reetz wrote:

I have consistently used the 24-hour clock format, with automatic date and time setting, but not automatic time zone setting. Recently (macOS 15.3.1), the clock (and all its dependencies, such as in the calendar) switched to AM/PM format. In the General → Date & Time settings, the “24h” option was disabled. Setting it back to “24h” did not make any difference (except for the “search” icon briefly disappearing from the top bar for a second, but no time change occurred). Restarting the Date & Time settings, adding a new user, starting in safe mode, and reinstalling the operating system all yielded the same result: the system refuses to use the 24-hour clock setting. Is there a specific system file that requires editing or deletion to resolve this issue? This is quite frustrating.

make changes to your settings and test... then come back to your preferred settings and compare your results.

—A SafeBoot Start up your Mac in safe mode - Apple Support will sort many anomalies

then reboot as normal and test. Caches get rebuilt automatically.

—You can try from the Terminal.app copy & paste:

sudo rm /var/db/timed/com.apple.timed.plist

this will remove the corrupt plist.

(note—your psswd will not echo on screen type it in anyway, use the enter\return key to proceed. Quit Terminal.)

Reboot and compare your results.

— The current stable release of Sequoia including bug fixes, security updates is macOS 15.3.2

Keep your Mac up to date - Apple Support

Keep your Mac up to date - Apple Support

—you can file a bug report / submit your Apple Feedback here: Product Feedback - Apple