Fifty-eight minutes is a generous class period with two minutes you can spend on the attendance you forgot. Do that first. Hunting names at the end makes the alarm feel like a punishment.
The rest is one activity with a product, not a sequence of announcements. 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 58 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 58 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. The rest is one activity with a product, not a sequence of announcements.
What is 58 minutes actually good for?
Fifty-eight minutes is a generous class period with two minutes you can spend on the attendance you forgot.
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.