Twenty-four minutes is just under a Pomodoro, for the afternoon when you cannot protect a full twenty-five. It divides into three eights: learn, try, correct.
Use the same alarm you use for twenty-five so your body learns the sound as 'stand up.' Press space to start or pause, R to reset, and F for fullscreen. The screen can stay awake after you start, using the wake lock your browser allows. Pick the alarm before you begin. A bell, a chime, a beep, or a short digital pulse all come from the Web Audio API, so there is no sound file to wait on.
This page is the 24 minutes countdown and nothing else is running underneath it. Related lengths are linked under the guide if you want a neighbor, and the timer itself stays above the fold.
Questions
How do I run a 24 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Use the same alarm you use for twenty-five so your body learns the sound as 'stand up.'
What is 24 minutes actually good for?
Twenty-four minutes is just under a Pomodoro, for the afternoon when you cannot protect a full twenty-five.
Will the countdown drift if I switch tabs?
No. The end is a timestamp. The display is the gap between that timestamp and now, so a background tab does not lose seconds the way a ticking counter would.