Seven hours is a workday without the lunch you hope you will take. The timer will not make you eat. Use it for a process that truly takes seven hours, like a cure or a charge, not as a motivational poster.
Notification permission is optional and only asked after you click. For a seven hour wait, the notification is the point. 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 7 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 7 hours timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Notification permission is optional and only asked after you click. For a seven hour wait, the notification is the point.
What is 7 hours actually good for?
Seven hours is a workday without the lunch you hope you will take. The timer will not make you eat.
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.