Ten hours is a long process timer: a charge, a proof, a download you do not trust the vendor's estimate for. Plug the computer in. A laptop asleep in a bag will not show you the end, even if the timestamp math is right when you return.
The alarm fires when the tab can run. Come back before you need the exact second. 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 10 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 10 hours timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. The alarm fires when the tab can run. Come back before you need the exact second.
What is 10 hours actually good for?
Ten hours is a long process timer: a charge, a proof, a download you do not trust the vendor's estimate for.
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.