Stopwatch and timer

How it works

The display reads the clock with performance.now(), which counts in milliseconds from a fixed point and is not affected by the wall clock changing. While running, the reading is the accumulated time from earlier runs plus the time since the current start, so pausing and resuming never loses or double-counts a millisecond. The display refreshes on a short interval that recomputes the total instead of adding ticks, so a throttled or hidden tab still shows the right time when you come back. A lap stores the reading at the moment you press it.

Worked example

Start the stopwatch, wait about a second and press Lap: the first lap shows the elapsed time at that moment. Press Lap again after another two seconds and the second lap reads about three seconds in total, because laps record the total running time, not the gap. Pause and the reading freezes exactly where it is; reset returns everything to 00:00.00 and clears the laps.

Questions and answers