Countdown timer

Pick a length, press start, and it chimes at zero. Presets cover the common ones; the slider goes up to two hours.

The timing is read from a monotonic clock each frame rather than counted up by adding one second per interval tick. That distinction is invisible for two minutes and obvious over an hour: interval ticks are only approximately on time and their error accumulates, so a naively-built timer runs minutes late. Reading the clock directly means the display is right the instant you look at it, even after the tab has been in the background — though a backgrounded tab is throttled, so the chime itself can arrive late even when the number is correct.