Overview
Timing things well turns out to be a small engineering problem. Browser clocks can be throttled in background tabs, ordinary timers drift, and a stopwatch that loses half a second a minute is worse than no stopwatch. This one reads elapsed time from a monotonic high-resolution clock, so pauses, tab switches and system-clock adjustments cannot corrupt the measurement.
Beside the stopwatch sits a countdown timer with a WebAudio alarm — the beeps are synthesised live, so no audio files are needed. Both instruments pause and resume cleanly, laps stack newest-first with jitter-free tabular numerals, and the digits stay big and readable at a glance.
What makes a stopwatch trustworthy
Trust comes from the clock source. Ordinary scripts measure time by counting timer ticks, which browsers happily slow down in background tabs to save battery — your stopwatch keeps “ticking” but the ticks get sparse. A monotonic high-resolution clock instead stamps the start moment and derives elapsed time by subtraction, so throttling cannot touch the result.
Precision is the second half: hundredths of a second are the practical limit of human reaction time anyway, and displaying them with tabular numerals — digits that never change width — stops the readout from jittering as it runs. That is why the display looks rock-steady even at full sprint.
Features
- Hundredth-second stopwatch on a monotonic high-resolution clock.
- Lap capture with newest splits on top and jitter-free tabular numerals.
- Countdown with alarm: set minutes and seconds, get a triple beep at zero.
- Pause and resume on both instruments without losing time.
- Clock-derived elapsed time, immune to background-tab throttling.
- WebAudio alarm generated live — no audio files needed.
- Minimal chrome: big readable digits, three buttons per mode.
- Private and offline-ready once the page loads.
Benefits
- Actually accurate. The clock source survives throttling and tab switches.
- Laps without fuss. Splits stack up, newest first.
- Alarm included. The countdown ends with sound, not silence.
- Glanceable. Big steady digits you can read mid-sprint.
Who can use it
Runners and swimmers timing intervals, cooks who need a reliable kitchen countdown, students running Pomodoro-style focus sessions, speakers rehearsing to a time limit, and board-game groups settling “whose turn is taking forever”. Lab demonstrators get a nice example of why clock sources matter.
How to use
- Choose the Stopwatch or the Countdown tab.
- On the stopwatch: press Start, Lap for splits, Stop to finish.
- On the countdown: set minutes and seconds, then Start.
- Pause and resume either instrument at any time.
- At zero the alarm sounds — press Stop to silence it.
Frequently asked questions
Why do some online stopwatches drift?
Because they count ticks instead of measuring time. Browsers throttle timers in background tabs to save battery, so a stopwatch built on setInterval falls behind whenever you switch away. This one stamps the start on a high-resolution monotonic clock and computes elapsed time by subtraction, which throttling cannot affect — the reading stays correct no matter what the tab was doing.
How accurate is a hundredth of a second, really?
More precise than the person pressing the button. Human reaction time to a visual cue is around two-tenths of a second, so hundredths are plenty for training, cooking and presentations. For Olympic photo-finishes you need dedicated hardware — but for everything else, this is honest precision. For everything else humans actually time by hand, this is honest precision with margin to spare.
Will the alarm sound if the tab is in the background?
The countdown itself stays accurate in a background tab because it is clock-derived. Whether the beep plays audibly depends on your browser's autoplay and background-audio policies — most browsers allow audio after you have interacted with the page, which pressing Start counts as. Keep the tab's sound unmuted to be safe. Keep the tab unmuted and your device volume up to be safe.
What is a monotonic clock?
A clock that only ever moves forward at a steady rate, unlike the wall-clock time your operating system shows — which can jump when NTP syncs or daylight saving changes. For measuring durations, monotonic is the right tool; for telling the time of day, the wall clock is. This stopwatch uses the monotonic one. This stopwatch uses the monotonic one, so its readings cannot be warped by a mid-session time adjustment.
Is anything recorded about my timings?
No. Your laps, splits and countdown durations exist only in the page's memory while the timer is running. Nothing is uploaded to a server, written to storage, or remembered between visits. Each session starts with a clean slate — refresh the page and every trace of your timings is gone, which is exactly how a simple timing tool should behave.
Conclusion
Good timing is a solved problem once the clock is honest. Start the stopwatch, trust the hundredths, capture the laps that matter, and let the countdown's alarm worry about the ending. The only thing left to measure is how well you used the time.