How to accomplish 24-hour clock on lock/login screen in macOS?

THAT IS IT!

Sorry for screaming boldly, but I had to let it out. 😉 I had my Lock screen on „Username and Password“ for decades and starting with Tahoe this no longer works the way it did. The iMac I compared my MBP to is set to show „User List“ and I would never have guessed that this could be the reason for its 24h-glory.

In hindsight it kinda makes sense, because this way my user account is already selected and its 24h setting applies. I think macOS should keep the last used clock format and not default to 12h. Will file a bug report on this.

Thank you so much, MichMart!

In my case, I eventually discovered that wherever I had the 12-hour format, I had “Username and Password” selected under “Lock Screen” in System Settings. After changing it to “User List,” I now have the correct 24-hour format on the login (or boot) screen.

So far, I've tested it on macOS 27.01 and macOS 27.2 B3. My default language settings are Czech.

I'm using FileVault by default, I haven't even turned it off.

Starting with macOS 26 Tahoe, FileVault is enabled by default. All Macs that can run Tahoe or later have the internal storage encrypted at the hardware level. FileVault increases security by incorporating the user's login password into the cryptographic key. Practically, that means that after a reboot/startup and before you enter your login password, the Mac is not able to access user account settings or the internet. 

When FileVault is enabled, a Mac boots by stopping at an early pre-boot authentication screen to decrypt the startup disk before loading the main operating system or network services. 

Changes to the boot sequence with FileVault enabled:

• Early Password Prompt: You see a special FileVault unlock screen immediately after the hardware loads, before normal user services or Wi-Fi connections start.
• No Network Access Yet: Because network stacks require user-space services to run, the Mac cannot connect to standard Wi-Fi networks until you enter your password to unlock the drive.
• Decryption Unlocks the OS: Entering your account password provides the key to decrypt the drive, allowing the Mac to hand off control and finish booting into the standard macOS interface

You can disable FileVault in System Settings > Privacy & Security > FileVault (near the bottom of the settings pane) – click the setting and toggle FileVault off.

With FileVault off, the boot sequence will be 'normal' because user settings and network access will be available prior to entering the login password. Keep in mind that Apple enables FileVault by default for a good reason – to protect your Mac. If someone gains physical access to your Mac and you do not have FileVault enabled, it is a simple matter for them to access the data stored on your internal drive, either by changing your login password or via the Recovery OS. With FileVault enabled, your login password is required to access your internal storage. 

Given the enhanced security, I recommend leaving/turning FileVault on. There is no ‘penalty’ in terms of performance, the data encryption/decryption is handled by dedicated hardware and is happening regardless of FileVault status, since that merely changes the key used for encryption. 

More info on FileVault:

Protect data on your Mac with FileVault – Apple Support

Intro to FileVault – Apple Support

Björn Herrmann wrote:

I just tried it and disabled FileVault. Then I switched from 24 to 12 hours mode and back, tried it with a reboot and just a logout. Didn’t help and it’s still stuck at 12 hours. Any additional ideas?

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

Does a quick disk repair before it fully boots up, and certain system caches get cleared and rebuilt, third party system modifications and system accelerations are disabled temporarily.

Login and test. Reboot  as normal and test.  Caches get rebuilt automatically.

Björn Herrmann wrote:

Safe Booting did not help either.

To be proactive you can file a bug report / submit your feedback here: Feedback - macOS - Apple  

further—

Call Customer Support  (800) MY–APPLE (800–692–7753)

or on line  https://getsupport.apple.com/

or call  AppleCare Support at 1-800-APLCARE (800-275-2273)

Outside the USA—Contact Apple for support and service by phone

See a list of Apple phone numbers around the world

Contact Apple Support - Apple Support 

Thanks, but I am using FileVault since it gained full disk encryption in Mac OS X 10.7. I even remember its 1st incarnation which only encrypted the user’s home directory in an image. I never turned it off since then and I always had the 24-hours clock on the lock screen for years while Mac OS X was renamed to OS X and later to macOS.

It only began with the update from Sequoia to Tahoe and persists in Golden Gate that I get to see the 12-hours clock meaning that I get this at

a) Preboot (FileVault)

b) Login Screen when having logged out

c) Lock screen when using Fast User Switching

So practically it’s wrong everywhere except for the menu bar clock. I just checked with an older Intel iMac 2020 (Tahoe) and it displays the time in 24-hours format.

For example right now it’s 00:25 (past midnight) and if I bring up the login screen via the Fast User Switching menu bar item

• my M1 MBP shows: 12:25

• my Intel iMac shows: 00:25

Björn Herrmann wrote:

Starting with macOS 26 Tahoe the clock of my login screen and lock screen was suddenly in 12-hours instead of 24-hours. This bugged me for a year now and I thought with macOS 27 Golden Gate’s polishing this would be corrected as were the hideous corner radii. But sadly this wasn't the case.

What I’ve tried so far:

• Enabled the root user and set it up as German and 24-hours

• sudo languagesetup to German in my main admin account as well as the root user

• Setting it to 12-hour mode and back to 24 in System Settings

• sudo defaults delete /Library/Preferences/.GlobalPreferences.plist AppleICUForce12HourTime (the key was not even there)

• sudo defaults write /Library/Preferences/.GlobalPreferences.plist AppleICUForce24HourTime -bool true

Nothing worked so far so I would really be glad if someone knows how to get my 24-hours back…

I have never used or had the FileVault turned on...

No issue with the 24 hour clock in Tahoe 26.7 in the menubar / lock screen / log in screen.

To be proactive you can submit your feedback here: Feedback - macOS - Apple

or make changes to your FileVault settings and compare your results.

I just tried it and disabled FileVault. Then I switched from 24 to 12 hours mode and back, tried it with a reboot and just a logout. Didn’t help and it’s still stuck at 12 hours. Any additional ideas?

Safe Booting did not help either.

Today I reinstalled macOS 27 over my existing installation to no avail. It’s currently 23:34 and the lock screen shows 11:34. This is so annoying…