Online Stopwatch

Online Stopwatch

Provides Start, Pause, Lap and Reset controls with an HH:MM:SS.mmm display. It uses performance.now, a monotonic browser clock, so elapsed time is not changed by manual wall-clock or time-zone adjustments.

Display refreshes about every 31 ms, not every displayed millisecond. Millisecond digits report calculated elapsed time at the latest refresh; they do not imply one-millisecond measurement accuracy or a guaranteed 31 ms callback schedule.

Lap records the cumulative elapsed time since the first start, not the interval since the previous lap. Pausing preserves elapsed time, Start resumes it, and Reset clears both elapsed time and all lap records.

The Copy all action copies the visible lap list. If no elapsed time exists, Lap adds nothing. There is no split-time column, export file, countdown, sound, cross-device synchronization or persistence after navigation/reload.

All timing is local to the current tab. Browser scheduling, background throttling, device sleep and display refresh limit practical precision, so use calibrated timing equipment for sport certification, laboratory or other regulated measurements.