Show a 24-hour clock in the Mac menu bar

macOS can show the menu-bar clock in 24-hour time — 14:30 instead of 2:30 PM — with a single toggle. Turn it on in System Settings → General → Date & Time, or run one line in Terminal. Either way the clock changes the instant you flip it, and reverses just as fast.

The one-toggle way

1. Open System Settings → General → Date & Time.
2. Turn on 24-hour time.
3. Look up at the menu bar — the clock switches from AM/PM to 24-hour instantly. No restart, no logout.

This is the same toggle on macOS 13 Ventura, 14 Sonoma, and 15 Sequoia; only the exact wording of the switch shifts slightly between releases. It affects the clock in the top-right corner and the expanded clock you see when you click it. Turn the toggle back off and the AM/PM format returns immediately.

The Terminal way

If you'd rather not open System Settings — or you're scripting a fresh Mac setup — the menu-bar clock reads one preference key. Write it, then nudge the clock process to redraw:

# Show the menu-bar clock in 24-hour time
defaults write com.apple.menuextra.clock Show24Hour -bool true
killall ControlCenter

# Undo — back to 12-hour AM/PM
defaults write com.apple.menuextra.clock Show24Hour -bool false
killall ControlCenter

The killall ControlCenter line restarts the process that draws the clock so the change appears without a logout. If the clock briefly vanishes, give it a second — it repaints on its own. The Show24Hour key lives in the com.apple.menuextra.clock domain, which stores every menu-bar clock option, so the Terminal route and the Settings toggle are simply two doors into the same preference.

The menu-bar clock vs. the system-wide format

There's a subtle distinction worth knowing. The toggle above changes the clock in the menu bar. macOS also has a separate, locale-level 24-hour setting under System Settings → General → Language & Region → 24-Hour Time. That one governs how times are formatted more broadly — in some apps, in date-and-time strings, and in the formatting previews shown in that pane.

• Just want the corner clock in 24-hour? The Date & Time toggle (or the Terminal command) is all you need.
• Want 24-hour everywhere the system formats a time? Turn on Language & Region → 24-Hour Time as well. The menu-bar clock usually follows it too.

Turning on both is the belt-and-braces approach if you never want to see AM or PM again. Each is independently reversible, so there's no risk in flipping one, seeing what it changes, and flipping it back.

While you're in the clock settings

A 24-hour clock pairs nicely with a couple of other menu-bar tweaks. If you want the corner clock to tick precisely, see how to show seconds on the Mac clock — the same Date & Time area holds that switch too.