Twelve hours is a half day on the clock, a proof, or an overnight you will sleep through. If you will be asleep, use a device that stays awake and a volume that can reach you. Read the terms: this is not a safety alarm.
The digits stay correct across background time because they are computed from the end timestamp. 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 12 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 12 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 digits stay correct across background time because they are computed from the end timestamp.
What is 12 hours actually good for?
Twelve hours is a half day on the clock, a proof, or an overnight you will sleep through.
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.