Force a 24-hour clock on Mac regardless of region

If your region is set to somewhere that defaults to 12-hour time — the United States, for instance — the Mac keeps nudging your clock back to AM/PM whenever the locale format reasserts itself. The fix is the explicit override: turn on System Settings → General → Language & Region → 24-Hour Time. That flag forces 24-hour formatting on top of whatever your region would otherwise choose.

Region default vs. the explicit override

Every region carries a default time format. Pick United States and macOS assumes 12-hour AM/PM; pick United Kingdom or Germany and it assumes 24-hour. That default is why the clock can seem to "revert" — anything that re-applies the region format (changing the region, some updates, a new login on a managed Mac) pulls the format back to the regional default.

macOS gives you a dedicated switch that sits above that default. When 24-Hour Time is turned on in Language & Region, it forces 24-hour formatting regardless of which region you've chosen — so you can keep your US region (for currency, number formats, and app defaults) while still reading 15:00 instead of 3:00 PM.

Force it on

1. Open System Settings → General → Language & Region.
2. Turn on the 24-Hour Time toggle. The formatting previews in that pane switch to 24-hour immediately.
3. Leave your Region exactly as it is — the override wins over the region's default.

This is the setting that actually "sticks," because you're no longer relying on the region to supply 24-hour; you've asked for it directly. The toggle exists on macOS 13 Ventura, 14 Sonoma, and 15 Sequoia.

Pin the menu-bar clock too

The Language & Region override governs system-wide formatting. The corner clock has its own switch, so set both to be certain it never slips:

1. Open System Settings → General → Date & Time and turn on 24-hour time.

Or set the clock's preference directly:

# Force the menu-bar clock to 24-hour
defaults write com.apple.menuextra.clock Show24Hour -bool true
killall ControlCenter

# Undo the menu-bar clock override
defaults write com.apple.menuextra.clock Show24Hour -bool false
killall ControlCenter

With the Language & Region override on and the menu-bar clock set, both layers point the same way and there's nothing left for a region default to flip.

If it still reverts

• Check you didn't change the Region afterward. Switching regions re-applies that region's default and can turn the override display off — re-enable 24-Hour Time after any region change.
• On a work-managed Mac, a configuration profile can enforce a locale on login. If 24-hour keeps disappearing after you restart, ask whoever manages the device whether a profile is setting the format.
• Confirm both switches are on. It's common to set the menu-bar clock but forget Language & Region, then wonder why app timestamps still show AM/PM.

While you're in Language & Region

The same pane controls how dates are written, which is the other format that trips people up across locales. See how to change the date format on Mac to make dates as unambiguous as your new 24-hour clock.