Thirty-one minutes is a half hour plus the minute you spend writing the next step. Prime numbers make bad circuits and good deep-work blocks. Stay with one thread.
If people ask why not thirty, the answer is the extra minute is for the note, not for more work. 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 31 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 31 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. If people ask why not thirty, the answer is the extra minute is for the note, not for more work.
What is 31 minutes actually good for?
Thirty-one minutes is a half hour plus the minute you spend writing the next step.
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.