Eight hours is a workday or an overnight dough in the wrong direction. Know which one you meant. Start it when the process starts, not when you meant to start an hour ago.
If the device might restart, the running countdown is not stored as a server job. It lives in this tab. 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 8 hours 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 8 hours timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. If the device might restart, the running countdown is not stored as a server job. It lives in this tab.
What is 8 hours actually good for?
Eight hours is a workday or an overnight dough in the wrong direction. Know which one you meant.
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.