Why 12 minutes
Twelve minutes is the Cooper test — run as far as you can in twelve minutes, and the distance estimates your aerobic fitness. Developed by Kenneth Cooper for the US Air Force in 1968, it is still one of the most widely used field tests of cardiovascular capacity anywhere.
It will not drift while you do something else
The countdown is worked out from a fixed end time rather than counted down frame by frame, so putting the tab in the background, locking your phone, or switching apps does not cost you any of the 12 minutes. Browsers throttle background JavaScript to save battery, and that is exactly what makes most browser timers lose time. Press start, go and do the thing you are timing.