Online Stopwatch

A monotonic clock for duration, frames for the screen

Measuring duration and reading the time are different things. Date.now() returns the system time, which can jump forward or backward when the computer syncs with a time server or the user changes the clock. performance.now() returns milliseconds since the page loaded on a timeline that only moves forward, the right clock for a stopwatch, because a time correction mid-count won't ruin the measurement.

The displayed duration is always 'now minus the start instant', computed on demand from that clock, never a sum of ticks. That distinction matters: a stopwatch that adds up timer intervals accumulates lag, because each interval arrives a little later than scheduled and the error is never given back. Here, visual updates are scheduled by the browser (and may be throttled in hidden tabs to save battery), but the number recomputed when the tab comes back is still exact.

• performance.now(): monotonic time, never goes backward.
• Date.now(): system time, can jump with NTP.
• Duration = current instant minus start instant.
• Summing intervals accumulates lag; recomputing from the clock does not.

Elapsed, lap time and the two gaps that reveal pace

Imagine three laps recorded at 0:30, 1:05 and 1:32. The elapsed column shows exactly those values. Each lap's own time, the interval since the previous record, is 0:30 for the first, 0:35 for the second and 0:27 for the third. The average lands at 0:30.7, the best lap is the third and the worst is the second.

The two gap columns answer different questions. The gap to the previous lap shows whether you sped up or slowed down against the segment right before it, useful for spotting fatigue or a one-off mistake. The gap to the average shows whether that lap stands out from your typical pace across the whole session, useful for discarding an outlier. Standard deviation summarizes consistency: the smaller it is, the more even the laps were.

The CSV export carries all of that with formatted times and also raw milliseconds, so you can continue the analysis in a spreadsheet without converting anything back.

Counting down, without imposing a cycle structure

The timer mode counts down from a duration you set in hours, minutes and seconds, with shortcuts for common durations, and signals the end with a sound alert generated by the browser itself through Web Audio, with no sound file to download. The alert can be turned off, and there is a button to check the volume before you start.

It is deliberately just a timer: one duration, one end. If what you want is a focus routine with short and long breaks chained automatically, that is the Pomodoro technique and there is a dedicated tool for it on this site. Here the goal is a single interval, like an oven timer, a timed test or a speaking limit in a presentation.

How far to trust it and what gets saved

Browsers deliberately round the high-resolution clock as a defense against timing attacks, and how much they round varies with browser, platform and the page's isolation settings. That is why the tool measures the real resolution on your device and warns you when you ask for more digits than the clock delivers. For official competitions, use certified equipment and the rules of the sport.

Nothing is sent to a server: start, pause, lap, export and the countdown all happen entirely in the browser. Laps, the session name, preferences and the history of previous sessions live in the browser's local storage, so an accidental reload does not erase the measurement. That also means the data is tied to that browser on that device, and disappears if you clear the site's data.