Stopwatch & Timer

Two tools on one page: a stopwatch that counts up to hundredths with lap marks, and a countdown timer up to 99 hours that beeps when it reaches zero. Both start from a button press; nothing is stored and nothing is sent anywhere.

Neither one accumulates time, and that is what keeps them honest. The stopwatch records the timestamp at which you pressed start and then, on every redraw, computes the difference against the clock afresh — so a slow frame, a busy tab or a hundred missed ticks cannot make it lose a millisecond. The interval that drives the redraw is 31 ms, about 32 times a second, which is smooth to the eye but slower than the hundredths digit changes: the display genuinely skips values rather than counting every one. The countdown works the same way from the other end, storing the absolute instant at which it should finish; pausing keeps the remainder, resuming sets a fresh deadline.

The practical consequence is what happens in a background tab. Browsers clamp timers in hidden tabs to once a second or less, so the reading stalls while you are away and then jumps straight to the right value when you return — the elapsed figure is correct, only the animation was paused. The alarm is the one thing that can genuinely arrive late, since the tick that notices zero is the tick that fires it.

Two caveats worth knowing. The clock being read is the system clock, so if something corrects it while you are timing — an NTP step, a manual change — the reading moves by exactly that much; for durations that must survive that, a physical stopwatch is still the better instrument. And the countdown rounds its seconds up, so a five-minute timer shows 00:05:00 for the whole first second and reaches 00:00:00 only at the moment it fires. Laps are absolute readings at the instant you tapped, not the gaps between them. The alarm is three 880 Hz sine pulses — the A above the treble staff — synthesised through Web Audio, so there is no sound file to wait for.