Change your Mac clock to 24-hour or 12-hour time

Why it happens

Your Mac picks 12-hour or 24-hour time from your region when it's set up, so a Mac set up in the UK or most of Europe often starts on 24-hour time. The Date & Time switch overrides that for the whole Mac. Apps like Calendar and Mail follow the same setting.

How to fix it

1. Click the Apple menu in the top-left corner and choose System Settings.

2. Click General in the sidebar, then Date & Time, and turn 24-hour time on for 17:00 or off for 5:00 PM.

   General›Date & Time›24-hour time

3. Look at the clock in the top-right corner of your screen. It should show the new format within a second.

Still not fixed?

1. If the time is right but the menu bar clock hides AM and PM, that's a separate switch. Open Menu Bar settings, click Clock Options and turn on Show AM/PM.

   macOS 26 Tahoe Menu Bar›Menu Bar Controls›Clock Options

   macOS 27 Golden Gate Menu Bar›Menu Bar Controls›Clock Options…

2. If the switch is greyed out, your Mac may be managed by a school or company. Ask whoever set it up, because they can lock this setting.

3. If one app still shows the old format, quit it fully with Command-Q and open it again. Apps read the setting when they start.

4. If the clock keeps going back after a restart, contact Apple Support. A setting that won't stay is rare and worth a proper look.