Forty-two minutes is six sevens, a rotation length for centers that need enough time to matter. Seven minutes a center is short. Instructions have to be on the table, not in your head.
Use one sound for the rotation and a different sound at the true end so the last group does not pack up early. 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 42 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 42 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 one sound for the rotation and a different sound at the true end so the last group does not pack up early.
What is 42 minutes actually good for?
Forty-two minutes is six sevens, a rotation length for centers that need enough time to matter.
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.