Online Stopwatch

A naive web stopwatch adds 10 milliseconds every time a timer fires. That drifts badly, because browsers throttle timers in hidden tabs to about once per second, and Chrome can stretch chained timers to once per minute after a tab has been hidden for five minutes. Every missed tick is time the stopwatch silently loses.

This stopwatch never counts ticks. When you press Start it records a timestamp, and every time the display refreshes it subtracts that timestamp from the current one. If the tab sleeps for ten minutes, the very first frame after you come back shows the correct total. Laps work the same way: each lap stores the total at the moment you pressed L, and the lap time is the difference from the previous lap.

The timestamps come from performance.now(), not Date.now(). Date.now() reads the wall clock, which can jump forwards or backwards when your computer synchronises with a time server or when daylight saving starts. performance.now() is a monotonic clock that only ever moves forward from the moment the page loaded, so an NTP correction in the middle of a run cannot add or remove a second from your result. Browsers deliberately coarsen its resolution (to somewhere between 5 microseconds and 1 millisecond) to block timing attacks, which is still far finer than the hundredths shown here.

Sports timing settled on hundredths for a reason. At the 1972 Munich Olympics, Gunnar Larsson won the 400 m individual medley over Tim McKee by two thousandths of a second; afterwards swimming's governing body ruled that results would be decided to 1/100 s, because pool lanes cannot be built identically to within a few millimetres. Olympic athletics also publishes sprint results in hundredths, even though photo-finish cameras capture thousands of frames per second.

For a hand-operated stopwatch, hundredths are already more precision than your finger can deliver. Visual reaction time averages roughly a quarter of a second, and you react at both the start and the finish, so a hand-timed result is honest to about a tenth. Use the hundredths to compare your own laps against each other, where the reaction delays largely cancel out, rather than to settle a race.

Running intervals, rehearsing a talk slide by slide, timing each question in a practice exam, or checking how long every step of a recipe takes: press L at each checkpoint and the table lists the lap time and the running total. With two or more laps the fastest is marked green and the slowest red, so uneven pacing is visible at a glance. If you need to plan the same session across countries, the time zone converter handles the clock side.

While the stopwatch runs, the page asks the browser for a screen wake lock so a phone or laptop does not dim and lock in the middle of a long run. The lock is released the moment you stop. Browsers without the Wake Lock API simply keep their normal power settings; the timing itself is unaffected.